Apotheosis

Special restrictions: You must be level 20 with at least ten million experience, as well as 18 intelligence and 20 wisdom. The verses to this ritual take hours, and because it costs so much and only works if recited perfectly, you must spend 1d4 months memorizing them. Your unique symbol must be carved on the ground you are standing on.

You begin a ritual to transform yourself into a god. After the sacred ritual is complete, your symbol begins to emit a holy light. Then you drink the blood of the god and make a dc15 wisdom check. On a failure, your symbol engulfs you, killing you both. Not even the strongest magic can revive you. On a success the entire world shakes, and you become a god. Work with your dm to determine what you are the god of. 

Godly benefits: All of your stats increase by half their original value. (For example, if your charisma was 12, it is now 18). 

You can cast spells of level four and under your previous class could cast as cantrips.

All of your spells higher than that are cast as if they were using a spell slot two levels higher.

A modified version of wish is available to your use once per day. It has restrictions, albeit higher than normal, but no monkeys paw. (For example: if you wish for your strength stat to increase by 8 for a brief period of time, it won't target bad wording, but it might increase it by four instead). In short, the wish will do no ill wanted effects.

You get a sturdy and powerful castle in the Astral Plane. Holy or unholy soldiers defend it.