Your statement here is incorrect. The main spellcasting list would be crippled, the gain of Necrotic spells is equal.
Edit: Oh, and Realms, have you considered combining this with some of the Cancer Mage abilities to turn it into say a 6/8 casting prestige class? The Mother Cyst is just begging for the Cancerous Companion feature and the disease powers are cool and would work pretty well with the core concept, perhaps even intelligent carcenogens?