Dreadhorde Arcanist

Trample
Whenever this creature attacks, you may cast target instant or sorcery card with mana value less than or equal to this creature's power from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead.

Foundations Jumpstart

Trample
Whenever Dreadhorde Arcanist attacks, you may cast target instant or sorcery card with mana value less than or equal to Dreadhorde Arcanist's power from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead.

Time Spiral Remastered

Trample
Whenever Dreadhorde Arcanist attacks, you may cast target instant or sorcery card with converted mana cost less than or equal to Dreadhorde Arcanist's power from your graveyard without paying its mana cost. If that card would be put into your graveyard this turn, exile it instead.

War of the Spark Promos

War of the Spark

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.

If Dreadhorde Arcanist leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to check whether the card is still a legal target.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast the card, you do so as part of the resolution of Dreadhorde Arcanist's triggered ability. You can't wait to cast it later in the turn. Timing permissions based on a card's type are ignored, and the spell will resolve before blockers are declared.

The instant or sorcery card you target must have mana value less than or equal to Dreadhorde Arcanist's power immediately after Dreadhorde Arcanist has attacked. Any other abilities that trigger when creatures you control attack won't have resolved yet.