Once each turn, when you play an attachment and a spell, get the Muster bonus.
- Find more cards with Muster in its category page.
Spells include Spells and Fast Spells.
Attachments include Curses, Weapons, Relics, Relic Weapons, and Cursed Relics.
If you are able to play an attachment and spell on the enemy turn, you can get the Muster bonus during your opponent's turn.
Muster is limited to triggering once each turn, but you can trigger it across multiple turns. 
The Muster keyword was first revealed in Dire Wolf Digital's mechanic announcement article. 
- DWD News - Flame of Xulta: Muster (September 17, 2019)