Delirium is an ability word that becomes relevant when there are four or more card types among cards in your graveyard. Delirium decks therefore try to fill their graveyards with a varied assortment of lands, creatures, instants, artifacts, and other card types.
Mishra's Bauble and fetch lands can help add card types without much effort, while Unholy Heat becomes dramatically stronger once delirium is active. Ishkanah, Grafwidow provides another substantial payoff for reaching four types.
