Call and Dismiss Deity Spells

Call spells bring the avatar, god, or Great Old One before the caster. Casting this type of spell is perilous indeed, even for cultists. Only cult priests or desperate people should choose to call a Mythos deity. A handful are cited here. Both Call and Dismiss spells are likely to exist for every Great Old One and for every form of Mythos god.
A group can assist by adding power to Call spells, with the caster acting as the focus for the group. Everyone present expends 1 magic point. Those who know the spell can sacrifice any number of magic points (including burning hit points). The total of the magic points spent is the percentage chance the spell works. For each magic point spent, the group must chant for a minute, but never for more than 100 minutes. On a roll of 100 the spell will always fail with all magic points sacrifice in the spell lost.
The caster also loses 1d10 Sanity points in casting the spell. Everyone present loses sanity points if the deity appears. The Keeper plays the thing and must create some motivation for the thing's behavior. Mostly, when a Mythos deity arrives, it wants to stay and is usually hungry.

Call Deity Spell Requirements

As mentioned, each deity requires certain conditions to be in place for the Call spell to work. Keepers should use these as inspiration but not be constrained by them for two reasons. The first reason is fiction-based; no human fully understands the Cthulhu Mythos, so there may be more than one way to Call a Deity. The second reason is that experienced players will quickly latch onto the familiar elements and the mystery will be blown away.