Dragon Cultist

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.

Choose a Background is a variant of the partner ability. You may have two commanders if one of them is a legendary creature with the choose a background ability and the other is a legendary Background enchantment. Backgrounds and cards with choose a Background do not interact with cards which have any other partner 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.