There is a strict order in which Tomb Kings magic must be performed.
All Bound items not used by characters.
Icon Bearer: Bound Items
Tomb Princes: "My Will Be Done!" and Bound Items
Tomb Kings: "My Will Be Done!" and Bound Items
Hieratic Hierarchy - Liche Priests and High Liche Priests must use their Bound Items when it is their turn to perform their Incantations.
The Casket of Souls