There are FOUR god spells in existence. These spells are the ultimate spells and as such can only be used if your level 50, and have ALL base classes rank 10. Specifically mage and cleric.
There is 1 support god spell, 2 offensive god spells and 1 defensive god spell.
Oblivion has been revealed to be the weakest and in Wrath's possesion.
Each god spell can only be learned by one person, which means 1 of the 4 is taken. The other 3 are currently unknown and not revealed to be in any being's possesion.