When pleased, Mellis will
- replace up to 5 empty bottles and cans with full ones, or
- teach you about a god you do not yet know, or
- give you some money (with the amount dependant on your Wisdom).
When angry, he will spread bad rumors about you to the other gods, lowering your favor with all of them.
Mellis is one of the best (if not the best) gods in the game, as his prayer can result in a variety of powerful, permanent boons. Aside from free food and healing, he will sometimes give you ommel excretions, boosting your attributes. The money he gives you is also nothing to scoff at, especially early in the game.
Offering these items to Mellis at his altar will receive a greater bonus towards your favor with him:
or any item made of an aligned material. See the List of materials for more details.