A hellspawn collector
A keeper of souls –
Give him his due
He’ll accomplish your goal.

As a personal addition:
Masselin makes deals with people, promising them their desires in return for innocent souls. Trapped within the demon, the unfortunates are kept alive, their souls tortured for the pleasure he gets from their suffering. The demon seeks willing, trusting souls, delivered by the one who sealed the pact.

To Vanquish Masselin:

A potion to vanish
To rid his foul purpose,
His power infernal,
Are boiled with these ingredients
A solution of Myrrh bark,
Mixed with Elecampane
Along with ---- Brown Rice
In Water to finish.
Combine those together
Blend them at once
Contribute Copper Leather
Your task is almost done.
The Demon must drink this
To relinquish his victims
To conquer his final breath
Is his -----------.