When pleased, Mortifer will recognize you as his champion and grant you a "heavy weapon of pure corruption".
Subsequent prayers will get you a temporary Ethereal status effect.
"I aM PlEaSeD By tHy sQuIrMiNg, WoRm! WaLkEtH WiTh mE ThRoUgH ThE ShAdOwS As oNe oF ThE DeAd!"
When angry, he will smite you with a bolt of black energy, dealing high damage and reducing your AStr, Agi and End by 1 point. As all gods, Mortifer can also summon a hostile angel when extremely angry.
There are no fixed locations for Mortifer's altar; you may find it randomly in the dungeons.
If you can raise you piety with Mortifer high enough to become his chamion, Neerc Se-ulb is a very powerful and useful weapon that can be obtained relatively early in the game.
If you had no luck with altars, try polymorphing another altar and you may end up with a brand new altar of Mortifer.
Note however that you cannot become a champion of both Valpurus and Mortifer, so if you plan on acquiring the valpurium arms offered by Valpurus, you should refrain from praying to Mortifer.
Offering some items to Mortifer at his altar will result in a greater bonus towards your relation with Him. These include scythes, crystal skulls and any items made of his aligned Materials. Note that simple bones and skulls are ignored while more precious materials such as Ommel Bone or weapons and armor made of Bone can also be used as an offering.