Almost never, and the reason is structural: in the cleanup step, players do not normally receive priority at all. Cleanup does its housekeep...
Almost never, and the reason is structural: in the cleanup step, players do not normally receive priority at all. Cleanup does its housekeeping — the active player discards down to their maximum hand size, damage wears off, and 'until end of turn' effects end — and the turn simply ends. No priority means no window to cast instants, which surprises players who planned to spend leftover mana after discarding.
The example card, read exactly
Necropotence — Enchantment (mana cost {B}{B}{B}):
Skip your draw step.
Whenever you discard a card, exile that card from your graveyard.
Pay 1 life: Exile the top card of your library face down. Put that card into your hand at the beginning of your next end step.
Rules answers live and die on exact wording, which is why every card named on this page can be checked in the MTGBrew card database before you rely on it at a table.
How it plays out, step by step
The exception is real, though narrow: if anything triggers during cleanup, or a state-based action does something, the active player receives priority after those resolve — and with priority in the cleanup step, spells and abilities become legal again. The classic engine for this is a hand full of cards plus an effect that triggers when you discard: discard, trigger goes on the stack, priority opens, and suddenly the cleanup step is a working window — once. After that round of priority, there is another cleanup step, and the cycle repeats until a cleanup passes clean. Build around it deliberately if you want it; do not expect it by accident.
When you build around interactions like this, the MTGBrew AI deck builder keeps the list honest — and the MTGBrew Learn guides collect more of these plain-English rules walk-throughs in one place.
If this topic is on your bench, keep going with Spellslinger Deck Tech: How the Archetype Works and Wins in Commander — it covers the neighbouring decisions from a different angle, and the two pieces together give you a fuller picture than either one alone.
The short version
The exception is real, though narrow: if anything triggers during cleanup, or a state-based action does something, the active player receives priority after those resolve — and with priority in the cleanup step, spells and abilities become legal again. Everything else in this article is the working-out — and at a real table, the working-out is what settles the argument, so walk the steps in order and let the text of the cards involved make the final call.
Keep reading
Keep reading: Spellslinger Deck Tech: How the Archetype Works and Wins in Commander and Precon Upgrade Guide: Coram, the Undertaker — What to Cut, What to Add, by Role.
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