Champion
The Champion is one of the fighter archetypes from the Player's Handbook (PHB), found on page 72. To make this build as simple as possible, we will not use any Feats, and we will make it a human. For ability scores, we will use the standard array given on page 13 of the PHB. This fighter will be designed to be decent at offense and defense, and will be based around using heavy armor and having a high strength score.
A human with the standard array starts with scores of 16, 15, 14, 13, 11, and 9. For our Champion, we will assign them as follows: 16 STR, 11 DEX, 15 CON, 13 INT, 14 WIS, and 9 CHA. For a background, we will select Sailor from page 139 of the PHB, which gives us the Perception and Athletics skills, both of which are valuable for survival in The Arena. From the fighter skills, we will select History and Insight, which can be useful in identifying monsters and their weaknesses and solving puzzles, respectively.
A fighter gets to start with a Fighting Style (PHB 72). To keep everything as simple and survivable as possible, we will take the Defense option. This gives the character a +1 to its AC, so it's not something you will have to remember in the course of play. You can just figure your AC once, write it on your sheet, and that's that.
Advancement
The Arena has different starting levels depending on when you start. New players might start at 1st level, or they might start as high as 11th level, if the game is in the third tier or above (PHB 15). At 1st level, the character will start with chain mail, a shield, a longsword, a light crossbow, 20 bolts, and an explorer's pack. Details on all of these items can be found in the Equipment section (PHB 143).
If the character starts at 5th level or higher, it can start with splint instead of chain mail. If it starts at 11th level or higher, it can also start with a +1 longsword.
The main choice you have to make when you gain levels is how to use your ability score improvements. Fighters gain more ability score improvements than other classes, getting one at 4th, 6th, 8th, 12th, 14th, 16th, and 19th levels. At 4th and 6th level, we will increase STR to 18 then 20. At 8th level, we increase CON to 16 and CHA to 10. At 12th and 14th, we take CON to 16 then 20. At 16th, we take DEX to 12 and INT to 14. Finally, at 19th, we take WIS to 16. We end with all even ability scores, which means our attribute assignment is as efficient as it can be. WIS ends up being our best mental stat, which is good, since so many saving throws target WIS, and since Perception, one of the most useful skills, is based on WIS. Both our STR and CON are at the maximum, to increase our melee offense and give us a lot of HPs.
The only other choice you will have for character customization as you advance is that the Champion gets a second fighting style at 10th level. Dueling is the simplest option, and is what is recommended for most players. It gives a flat +2 bonus to one-handed melee weapon attacks. You can, again, record it on your character sheet in the weapon damage section, and that's that. More advanced players may want to select the Protection style or something else, but Dueling is likely the best choice for almost anyone playing a Champion fighter.
Tactics
The main reason that this build is included among the straightforward options for new players is that it is very simple to play. It is based entirely around going up to the enemies and hitting them with your sword, in the hope that your good armor class and high HP total will allow you to withstand any counterattacks. From 1st to 4th level, you get one attack per round when you take the Attack action. At 5th level this increases to two attacks, at 11th level this increases to three attacks and, finally, at 20th level it tops out at four attacks. This means that the Attack action is going to be your bread-and-butter in combat.
The main things you will have to remember as a Champion in combat are that, at 3rd level you have the Improved Critical feature, meaning you score a critical hit on an unmodified roll of 19 or 20 on a d20 instead of just a 20. At 15th level, this increases with the Superior Critical feature, where a roll of 18 also scores a critical hit. The other thing to be sure to remember is that you have the Indomitable feature starting at 9th level. This lets you reroll a saving throw that you fail. Generally speaking, you should use this the first time you fail a saving throw, unless it is a spell that just does damage and you have a lot of HPs left. If that's the case, you may want to save it for something more debilitating. At 13th level, you get to use this feature twice. At 17th, you get to use it three times. At 13th level and beyond, you should probably just use your Indomitable feature the first time you fail any saving throw.
The most potent ability that fighters have is their Action Surge. In story and campaign-based D&D, it is often a good idea to wait to use it at a pivotal moment. In The Arena, however, it is usually best to use it right away. If you have the opportunity to use it in your first round to get an additional Attack action, go for it. As soon as that opportunity presents itself, it's usually worth taking it. It might let you clear our some lesser enemies, or it might let you put a severe hurting on a tougher one. The other signature feature to use is the Second Wind. Generally speaking, as soon as you start your turn and your HP total has dipped below your maximum by at least 10 + your level (the maximum you can recover from a Second Wind), you should use your Second Wind. But, you may want to consult the party healer first to make sure there isn't some major healing effect coming along before your next turn that will let you save your Second Wind for another time.
A human with the standard array starts with scores of 16, 15, 14, 13, 11, and 9. For our Champion, we will assign them as follows: 16 STR, 11 DEX, 15 CON, 13 INT, 14 WIS, and 9 CHA. For a background, we will select Sailor from page 139 of the PHB, which gives us the Perception and Athletics skills, both of which are valuable for survival in The Arena. From the fighter skills, we will select History and Insight, which can be useful in identifying monsters and their weaknesses and solving puzzles, respectively.
A fighter gets to start with a Fighting Style (PHB 72). To keep everything as simple and survivable as possible, we will take the Defense option. This gives the character a +1 to its AC, so it's not something you will have to remember in the course of play. You can just figure your AC once, write it on your sheet, and that's that.
Advancement
The Arena has different starting levels depending on when you start. New players might start at 1st level, or they might start as high as 11th level, if the game is in the third tier or above (PHB 15). At 1st level, the character will start with chain mail, a shield, a longsword, a light crossbow, 20 bolts, and an explorer's pack. Details on all of these items can be found in the Equipment section (PHB 143).
If the character starts at 5th level or higher, it can start with splint instead of chain mail. If it starts at 11th level or higher, it can also start with a +1 longsword.
The main choice you have to make when you gain levels is how to use your ability score improvements. Fighters gain more ability score improvements than other classes, getting one at 4th, 6th, 8th, 12th, 14th, 16th, and 19th levels. At 4th and 6th level, we will increase STR to 18 then 20. At 8th level, we increase CON to 16 and CHA to 10. At 12th and 14th, we take CON to 16 then 20. At 16th, we take DEX to 12 and INT to 14. Finally, at 19th, we take WIS to 16. We end with all even ability scores, which means our attribute assignment is as efficient as it can be. WIS ends up being our best mental stat, which is good, since so many saving throws target WIS, and since Perception, one of the most useful skills, is based on WIS. Both our STR and CON are at the maximum, to increase our melee offense and give us a lot of HPs.
The only other choice you will have for character customization as you advance is that the Champion gets a second fighting style at 10th level. Dueling is the simplest option, and is what is recommended for most players. It gives a flat +2 bonus to one-handed melee weapon attacks. You can, again, record it on your character sheet in the weapon damage section, and that's that. More advanced players may want to select the Protection style or something else, but Dueling is likely the best choice for almost anyone playing a Champion fighter.
Tactics
The main reason that this build is included among the straightforward options for new players is that it is very simple to play. It is based entirely around going up to the enemies and hitting them with your sword, in the hope that your good armor class and high HP total will allow you to withstand any counterattacks. From 1st to 4th level, you get one attack per round when you take the Attack action. At 5th level this increases to two attacks, at 11th level this increases to three attacks and, finally, at 20th level it tops out at four attacks. This means that the Attack action is going to be your bread-and-butter in combat.
The main things you will have to remember as a Champion in combat are that, at 3rd level you have the Improved Critical feature, meaning you score a critical hit on an unmodified roll of 19 or 20 on a d20 instead of just a 20. At 15th level, this increases with the Superior Critical feature, where a roll of 18 also scores a critical hit. The other thing to be sure to remember is that you have the Indomitable feature starting at 9th level. This lets you reroll a saving throw that you fail. Generally speaking, you should use this the first time you fail a saving throw, unless it is a spell that just does damage and you have a lot of HPs left. If that's the case, you may want to save it for something more debilitating. At 13th level, you get to use this feature twice. At 17th, you get to use it three times. At 13th level and beyond, you should probably just use your Indomitable feature the first time you fail any saving throw.
The most potent ability that fighters have is their Action Surge. In story and campaign-based D&D, it is often a good idea to wait to use it at a pivotal moment. In The Arena, however, it is usually best to use it right away. If you have the opportunity to use it in your first round to get an additional Attack action, go for it. As soon as that opportunity presents itself, it's usually worth taking it. It might let you clear our some lesser enemies, or it might let you put a severe hurting on a tougher one. The other signature feature to use is the Second Wind. Generally speaking, as soon as you start your turn and your HP total has dipped below your maximum by at least 10 + your level (the maximum you can recover from a Second Wind), you should use your Second Wind. But, you may want to consult the party healer first to make sure there isn't some major healing effect coming along before your next turn that will let you save your Second Wind for another time.