Source 1 (Enchantments that turn into enchantment creatures - "Vision Design didn't envision this as a whole mechanic but as just individual cards. Set Design would then turn it into a mythic rare cycle [...]") - Source 2 (Cellarspawn - "[...] the overlords, which Valgavoth deliberately created to help enact his will and carry out occasional targeted campaigns of terror. There's one overlord for each zone of the House.") - Source 3