Seregios, Swooping Intruder

Flash, flying, haste
Whenever Seregios enters the battlefield during combat, you may have it enter under an attacking
player's control tapped and attacking. At the end
of combat, Seregios' owner gains control of it.
Whenever a player is dealt damage by Seregios,
that player gains a bleed counter. Whenever an opponent casts a spell, they lose a life for each
bleed counter they have.