Dragon Cultist

Once the game begins, your two commanders are tracked separately. If you cast one, you won’t have to pay an additional {2} the first time you cast the other. A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background won’t usually be a creature anyway).

If you control a Background that grants an ability to commander creatures you own, and you own more than one commander creature, each of them will have that ability.

The triggered ability granted by Dragon Cultist looks for the total amount of damage dealt by a single source, even if that damage wasn't dealt all at once or wasn't dealt to the same player or permanent. For example, if an attacking 3/3 creature with double strike deals 3 damage to one blocker in the first combat damage step and 3 damage to another blocker in the second combat damage step, then it has dealt a total of 6 damage and will cause this ability to trigger.

You need to control the source that dealt damage only at the time that the damage was dealt. If it dies or changes control after that point, this ability will still trigger at the beginning of your end step as long as you control a commander creature with the ability.

If your commander loses the choose a Background ability or stops being a Background during the game, as appropriate, it is still your commander.

You can choose two commanders that are the same color or colors.