From Dungeons of Dredmor Wiki
A dullness afflicts your mind due to contact with dark magics.
Debuff: -2 for 93 turns
Lingering Dullness is a detrimental effect (Curse) that affects characters that cast spells such as Nightmare Curse.
It is a side-effect of spells cast by characters with the Necronomiconomics skill tree. It can also affect characters that drink from a Fountain.