After entering the portal, travel through the slime dimension until you find Lyphn.
What do I do about Lyphn?
You can't defeat it, but you can annoy it.
There are more than a dozen ways to bother Lyphn. The garnet and turquoise eyestones are especially useful. But you can't use the same trick more than once.
After you've broken Lyphn's concentration a couple of times, you'll be able to grab the hagfish amulet (and the opal eyestone) from Terlissa.
Bother Lyphn some more, and you can grab Terlissa herself.
Take the hagfish amulet and opal eyestone back through the portal. Whether you take Terlissa is up to you.
When you're back on your native side of the portal, equip the hagfish amulet and opal eyestone, and point at the portal to close it. Then get back on the boat to finish the story.
Return to the OFFICIAL PLAYER'S GUIDE.
Or check out a complete table of spells, if that interests you.