Info about vanilla special effects used to modify enemy stats.
These are GENERALLY applied per-area, but there are exceptions (especially in certain areas).
Gravelord Enemy multiplier is applied along with a normal stat multiplier.

Special Effect General Areas Max HP Mult Damage Mult Defense Mult
7001 Undead Asylum 1 1 1
7002 Undead Burg 1.2 1.125 1
7003 Undead Parish, Lower Undead burg 1.4 1.25 1.27
7004 Depths 1.5 1.375 1.67
7005 Darkroot Garden (early) 1.5 1.5 1.8
7006 Firelink Shrine (skeletons) 1.6 1.6 1.93
7007 Catacombs, New Londo Ruins (early) 1.7 1.7 2.06
7008 Blighttown 1.8 1.8 2.2
7009 Sens Fortress 2 1.9 2.33
7010 Anor Londo 2.1 2 2.46
7011 Darkroot Garden (late), New Londo Ruins (late), Valley of Drakes 2.2 2.1 2.6
7012 Tomb of the Giants, Great Hollow/Ash Lake 2.3 2.2 2.73
7013 Demon Ruins (Not really though. LOT of variation) 2.4 2.3 2.86
7014 Painted World, Lost Izalith, Duke's Archives/Crystal Cave, DLC 2.5 2.4 3
7015 Kiln of the First Flame 2.7 2.5 3
7100 Gravelord Enemy 2 1.8 1.2