If a player has more cards in their hand than their maximum hand size during the cleanup step of that player's turn, that player discards until they have that many cards. A player's maximum hand size isn't checked at any time other than their own cleanup step.

If multiple effects modify your hand size, apply them in timestamp order. For example, if you put Necrodominance onto the battlefield and then put Spellbook (an artifact that says you have no maximum hand size) onto the battlefield, you would have no maximum hand size. However, if those permanents entered the battlefield in the opposite order, your maximum hand size would be five.

While Necrodominance is on the battlefield under your control, abilities that trigger whenever a permanent you own is put into your graveyard from the battlefield (for example, "When this creature dies…") won't trigger because cards and tokens are never put into your graveyard.

If a Necrodominance you control is destroyed by a spell you own, Necrodominance will be exiled and then the spell will be put into your graveyard.

If you discard a card while you control Necrodominance, abilities that function when a card is discarded (such as madness) still work, even though that card never reaches your graveyard. In addition, spells or abilities that check the characteristics of a discarded card (such as connive) can find that card in exile.