Summons one of the rift entities you control, if any. The summoning has a 100% success rate. Stronger entities and some rift tasks will speed up the 25 round cast time.

If the entity is summoned after being killed, it will appear with only one hit point.

The in game help reads:

No saving throw.
Casting time: 25 rounds.
Type of spell: neutral spell.
Affecting stats: int/wis.
Cast type: rift.
Spell point cost: 261.
You can use this spell to summon one of your rift entities. The time it takes
you to cast will decrease as all your entities grow in power. An entity will
disappear if you are not in the front row of the party formation, and suffer a
health penalty when you next summon it. Type 'gem entities' for a list of
available entities.
Syntax: cast summon rift entity at <type/name>
This spell uses 5 rift sparks.