Starspawn Grimoire is used to uncap , , or characters instead of using treasures. One grimoire is consumed to uncap one time.
Alternatively, Starspawn Grimoire can be used to uncap Eternals to 5★. Fully uncapping Eternals requires , which are required in large quantity during the process of recruiting an Eternal. Using a Starspawn Grimoire means avoiding the need to grind , players could also choose to grind Violet Trial, or to leech Xeno Clash Event raids to reliably obtain Rainbow Prisms. Violet Trial on Very Hard drops Rainbow Prisms somewhat reliably and should be considered during 1/2 AP for cost efficency. Xeno Clash raids on the other hand is not always available to the player due to being tied to events, but does drop rainbow prisms more reliably than Violet Trials. Whether or not to use a Starspawn Grimoire is up to the player's discretion.