Prismari, the Inspiration, and Twincast

Prerequisites

• Prismari, the Inspiration on the battlefield
• Twincast in hand
• available

Steps

1. Cast any other instant or sorcery by paying its mana cost
2. Holding priority, cast Twincast by paying its mana cost, targeting the instant or sorcery cast in step 1
3. Twincast's storm ability triggers, creating at least one copy of Twincast targeting the original Twincast
4. Resolve a copy of Twincast, copying Twincast targeting the original Twincast
5. Repeat step 4

Results

• Infinite magecraft triggers