Archfiend of Despair makes every point of opposing life loss more consequential. At each end step, every opponent loses life equal to the life they already lost that turn. Combat damage, burn, drain, and life payments can all contribute, while the Demon also prevents opponents from gaining life to recover.
A multiplayer drain effect can set up an especially strong end step. If each opponent loses several life during the turn, the Archfiend repeats that amount as additional life loss. The ability checks each opponent separately, so someone who lost no life does not lose life merely because another opponent took a large hit.
This is not a replacement effect that doubles every event immediately. The extra loss waits until the end-step trigger resolves, leaving a window for opponents to interact. Damage prevention can affect damage before it becomes life loss, but the end-step ability itself causes life loss rather than dealing damage. Effects that specifically care about damage will not automatically recognize that later loss. Eight mana makes reanimation or substantial ramp attractive. A deck with regular drain effects can benefit as soon as the next end step arrives, without needing the flying body to attack. Establish the Demon when life loss has already happened or when you can produce it promptly. That gives the card an immediate purpose and can turn an otherwise manageable turn of damage into a threat to several players at once.
