As long as your devotion to red and green is less than seven, Xenagos isn't a creature.
At the beginning of combat on your turn, another target creature you control gains haste and gets +X/+X until end of turn, where X is that creature's power.
Xenagos is a beatdown commander that doubles a chosen creature's power and grants haste every combat, turning fatties into one-shot kill machines. You ramp into a big threat, swing for commander-damage-level beatings, and close games fast before opponents stabilize. He stays out of harm's way as an indestructible enchantment until your devotion hits seven.
Doubles power AND grants haste at zero cost every turn, enabling explosive one-turn kills
Indestructible god that's hard to remove and dodges board wipes
Green/red gives access to the best ramp and the biggest creatures in the format
Commander damage and trample make oppressive blockers irrelevant fast
Heavily reliant on the buffed creature surviving—spot removal blanks your whole turn
No card advantage engine; can run out of gas in grindy games
Vulnerable to fog effects, edicts, and 'must attack' style punishment
Telegraphs the threat, inviting blockers and combat tricks against your single attacker
Legendary Enchantment Creature — God
