Rare Grimoires: 

At the Sinner's Corner save point/teleport/room
and the four rooms immediately surrounding it is
a phantom class enemy called the Dark Eye. After
a Dark Eye is defeated it may drop a Grimoire for
one of five different, rare, warlock class spells.
Those spells are Solid Shock, Lightning Bolt, 
Fireball, Vulcan Lance, and Dark Chant. These Rare
Grimoires are dropped very infrequently, and some
time is needed to get them.