+ 'Likhar the Gravespawn' morphed into 'Rotflesh Gorefeast' (Scavenger-based, high HP, low dmg, corpse eater, corpse pack, different color);
+ Better 'Magic spell type 1' sound;
+ Uber Diablo is immune to all but Lightning (lite-resistant);
+ Bloodskin wyrm lost its SC immunity, and also lost 10% of its HP;
+ Immortal King data adjusted;
+ Galahad's pack changed to Succubi (used to be Goat Lords);
+ Some DLVL 11 boss data adjustments;
+ Some CaveBoars didn't cause damage upon charging, now it's fixed;
+ Balthazar now accompanied by Vicious Imps;
+ Mud Golems: +25 AC;
+ Replacing Shadow Beasts with new Corrupt Order (Stalking Assassins) - use bow, sneak, retreat a lot, high AC, low HP, lite arrows, new GFX;
+ Unique soundset for Corrupt Order;
+ Shadow beast boss kicked from the game;
+ Cathedral monsters data adjusted again (for smoother balance), a lot of work done in this area... ;
+ Skeleton King is now resistant to Fire (fry the bastard if you're playing a Mage);
+ Greater Demon: subtype lowered;
+ Lazarus room in MP: some monsters replaced with others;
+ Lazarus level in SP: added 2 monsters, replaced some (this will be fun...);
+ Zavulon will usually stand guard with his pack outside of Lazarus room or keep the gate to Pandemonium (only in MP, as before);
+ Less HP and greater HP variety on Greater Demons;
+ Diablo's weakness changed to Fire, this stops him from becoming too hard, but his pack will be immune to Fire (use Lite), so beware;
+ Corrupt Order now always stand guard (appear) at the gateway to Hell (DLVL 13), they use faster retreat, and their arrows are deadly;
+ New boss on DLVL 13: Xorene the Lazarus Guard will lead a pack of Doom Knights and shoot Ligtning arrows;
+ Uber Diablo book speech improved;
+ A bit higher volume for Corrupts' attack sounds;
+ Zabulon now accompanied by Death Knights;
+ Ur-Annon now has a mob of Vampires;
+ The Vizier now has Dark Mages as his pack;
+ Warlord of Blood now appears in MP also, but now his escort is Death Knights;
+ Warlord's greeting speech improved;
+ Death Knights now attack faster than DLVL 13 Knights (14 > 12), but slower than Crypt Knights (10), which makes the balance better

upd:
the link below is the Music theme for Abyss...
TH Abyss theme