Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

If a card with persist is removed from the graveyard after it dies but before the triggered ability resolves, it won't be returned to the battlefield.

If a creature with persist that has +1/+1 counters on it receives enough -1/-1 counters to cause it to be destroyed by lethal damage or put into its owner's graveyard for having 0 or less toughness, persist won't trigger and the card won't return to the battlefield. That's because persist checks the creature as it last existed on the battlefield, and it still had -1/-1 counters on it at that point.

A nonmodal double-faced card enters with its front face up by default, unless a spell or ability instructs you to put it onto the battlefield transformed or allows you to cast it transformed, in which case it enters with its back face up.

In some rare cases, this permanent might transform while its triggered ability that allows you to pay mana to transform it is still on the stack. (For example, another effect might cause that ability to trigger an additional time.) In such a case, even if you pay mana as the triggered ability resolves, this permanent won't transform because it already transformed while the triggered ability was on the stack.

If you sacrifice Eirdu while casting a creature spell (to activate a mana ability, for example), the spell won't have convoke when you pay its costs unless it has convoke some other way.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.