SFS Standard Symbiotic Summon [free-action] Feat 5

Source Galactic Ancestries pg. 106
Prerequisites Hostless Entu
You can instantly merge with a creature you summon. If the next action you use is to Cast a Spell that summons a minion, you merge with the minion instead of summoning them. This works like animal form, except your form's Size, Speeds, AC, senses, attacks, and Athletics modifiers are identical to the summoned creature (you don't gain temporary Hit Points). If you're reduced to 0 Hit Points while merged with your summoned minion, the effect ends, and you return to your normal self with half as many Hit Points as you originally had when you first merged with your summoned minion.

Traits

Concentrate:An action with this trait requires a degree of mental concentration and discipline.
Entu:A creature with this trait is a member of the entu colony ancestry. They are telepathic fungal colonies that feed on emotion and imbue intelligence into willing hosts. An ability with this trait can be used or selected only by entu colonies.
Spellshape:Actions with the spellshape trait tweak the properties of your spells. These actions usually come from spellshape feats. You must use a spellshape action directly before Casting the Spell you want to alter. If you use any action (including free actions and reactions) other than Cast a Spell directly after, you waste the benefits of the spellshape action. Any additional effects added by a spellshape action are part of the spell's effect, not of the spellshape action itself.