Overlord of the Balemurk

Impending 5—{1}{B} (If you cast this spell for its impending cost, it enters with five time counters and isn't a creature until the last is removed. At the beginning of your end step, remove a time counter from it.)
Whenever Overlord of the Balemurk enters or attacks, mill four cards, then you may return a non-Avatar creature card or a planeswalker card from your graveyard to your hand.

Duskmourn: House of Horror

Impending 5—{1}{B}
Whenever Overlord of the Balemurk enters or attacks, mill four cards, then you may return a non-Avatar creature card or a planeswalker card from your graveyard to your hand.

Duskmourn: House of Horror Promos

"Impending N–[cost]" is a keyword that represents multiple abilities. The official rules are as follows: (a) You may choose to pay [cost] rather than pay this spell's mana cost. (b) If you chose to pay this spell's impending cost, it enters the battlefield with N time counters on it. (c) As long as this permanent has a time counter on it, if it was cast for its impending cost, it's not a creature. (d) At the beginning of your end step, if this permanent was cast for its impending cost, remove a time counter from it.

If an object enters as a copy of a permanent that was cast with its impending cost, it won't enter with time counters, and it will be a creature.

If you choose to pay the impending cost of a creature spell, it's still a creature spell on the stack. You can cast that spell for its impending cost only when you could normally cast that creature spell. Most of the time, this means during your main phase when the stack is empty.

If you choose to pay the impending cost rather than the mana cost, you're still casting the spell. It goes on the stack and can be responded to, countered, and so on.

You may return any non-Avatar creature card or any planeswalker card from your graveyard to your hand with Overlord of the Balemurk's last ability, not just one you milled while resolving that ability.