When I put down a summon sign something came up and said "disasters are gone after defeat of the gravelord servant.
That is the Gravelord Servant Conenant, they use the Eye of Death to curse 3 random worlds. If you find the place they droped the Eye of Death you can invade his world. See wiki link,
It just means prepare to die (F' up the gravelord servant). It also means that your world is cursed and it just gotten even harder than it already is.
I did not have a harder game though. I actually went through and killed a boss on one try?