Return to the pirate base in the third world with both Tycoon sisters to receive the summon.
World 3: Pirate Cove
After your entire party is reunited you should revisit the Pirates’ Hideout. When you stopped by to check on Boko after you first arrived in the merged world you didn’t have Faris with you. Of course the pirates want to know how their old captain is getting on, but there is a better reason to stop by the hideout than reminiscing with the drunken lugs.
As you venture inside, Faris becomes sidetracked. After a brief hallucinatory scene you gain a new summoning monster: Syldra.
There really isn’t a whole lot else to do in the cave. Many of its residents aren’t sober enough to talk coherently. There is a lot of hiccupping babble. You have better things to do, so go do them!