If a creature with undying dies with {Grave Betrayal} out, which takes effect?
Undying would go off first. {Grave Betrayal} creates a delayed trigger that goes on the stack at the beginning of the end step. Undying will have already resolved before Grave Betrayal goes off.
RESOLVED
That's a nice lol combo tho bc undying only saves them once. Add in {Corpsejack Menace} and you got yourself a party!!!
EDIT: whoa sorry lol. Misread and thought it worked for your own creatures. RTFC!!!!!