As long as your devotion to white and black is less than seven, Athreos isn't a creature.
Whenever another creature you own dies, return it to your hand unless target opponent pays 3 life.
Athreos punishes opponents every time your creatures die: they either lose 3 life or hand you the creature back. Build an aristocrats/recursion shell that sacrifices and reanimates value creatures repeatedly, draining the table to death while you reload your board. Mid-to-late game you flip Athreos into an indestructible beater once devotion hits seven.
Relentless recursion engine that makes every death a tax on opponents
Indestructible commander immune to most removal
Cheap, color-light commander that comes back regardless of board state
Excellent at grinding out attrition and resource wars
Death trigger is a 'may pay' choice opponents control, so life-rich tables blunt it
Athreos rarely attacks early due to the devotion-seven requirement
Vulnerable to exile-based removal and graveyard hate
Slower combo/control decks can outrace its incremental drain