Assuming that they bring forth a place of power or somehow get Erisu to a place of power, they also need a ritual to dispel him. The characters might already have a ritual of dispelling, the one from Illustrious Castle. If so, those words will work fine--but the symbolism will need to be Erisu’s symbolism.
If they find Elizabeth Mardel’s ritual of dispelling, that will work also, assuming they’ve been through enough to believe that the tree is a real force in the world along with Christianity.
If they have to make one up themselves, that’s also fine. The important bit is the symbolism--the actual sigil of Erisu and the secondary symbols in the circle--and calling on greater powers that not only exist but that the characters have some connection with.
If Erisu escapes, remember that rituals can be performed if they have something “deeply attached to” the target. Melody’s ghost is deeply attached to Erisu, because Melody was how Erisu was summoned.
It is unlikely that the adventurers will travel all the way to Brightwood Crossing during this adventure. But there is information there, information that they might run across before starting the adventure or after completing it. You might also include information in the goblin stronghold that leads to further adventurers for the characters.
Alan Mardel was captured in a trap that was meant to net Colonel Courlander. He was taunted by a demon, Erisu, summoned by a Night Priest.
This is where Joe Lakono turned Alan Mardel’s love for Melody into a curse on the House of Lisport.
The Goblin Mage used carnage at the battleground as a moment of power, and summoned Erisu. He used Alan’s love for Melody to send the demon to Lisport to possess her.
Today the battlefield is a field of lilies and poppies. But the great battle here fueled the moment of power that allowed the Goblin Mage to summon Erisu into Melody Lisport, using her fiancé Alan Mardel as the catalyst. Mardel led the Lisport armies into battle when Colonel Courlander was taken with fever.