Banned for... Oh fine...
First, make an offering of lots of blood and souls to the Old Ones...
Next, chant the Invocation of The Dead in their presence.
Then, make an offering of wine and grapes to gods of wisdom.
And finally, chant the Invocation of the Dead backwards in their presence (Or in their temples).
That should cause a temporary leak in the involved gods' and Old Ones' minds, giving you the knowledge you need to perform necromancy.