The Orb Guardians should turn friendly when you grab the Orb. After all, you're the new master of the damned thing, they should be doing what you say. Oh, and make it heavier so it's harder to apport. Or just make it un-apportable.
Yeah, it should give you some nice powers. Increased regeneration, increased mana regeneration, inVoke to summon friendly Orb Guardians, increased spell power and success rates, decreased exp gains (just like the robe of the archmagi), and for as long as you carry it, might, brilliance, and agility.
On the other hand, all monsters should be generated awake. Upon entering a level, all monsters that are asleep are awakened. This is better than constantly making noise since it doesn't kill stealth. Certain monsters get upgraded (Orcs turn into warlords, high priests, or sorcerers. Skeletons turn spectral. Hydras get 5 bonus starting heads. All ugly things turn into very ugly things. All slime monsters are titanic. All jellies/oozes are death oozes, azure jellies, or acid blobs. All crystal balls get turned into orbs of fire. All reptiles turn into dragons. All shapeshifters are glowing. All liches are ancient. All bats are vampires. All plants are oklob plants. There's more, this is just what comes to mind). Unintelligent monsters have a 1-in-HD chance of berserking every turn they see you with the orb. Intelligent monsters that do not cast spells should have a 5-in-HD chance per turn of choosing to cast berserk or not. Intelligent monsters that do cast spells have berserk and berserk other added to their spell list. The "in-HD" thing is so that weak monsters get turned into bigger threats by berserking rather than monsters with nasty powers not using them since they're berserked. Also, on D:1, the dungeon makes its final stand against allowing you to escape. D:1 with Orb, specifically near the entry vault you originally entered, should be the toughest challenge in the game, easily.
Pretty much just for flavor, but if the player's carrying the Orb and worships Beogh, ALL orcs should become followers. Who wouldn't join a guy carrying that kind of ancient artifact? I wonder how Nemelex would react if you sacrificed the Orb to him. I wonder how Lugonu would react if you carried the Orb into the Abyss and just dropped it. I wonder how Xom would react if you lost the Orb somehow. I wonder how Trog would react if you wielded the Orb and beat monsters to death with it. I wonder how Beogh and your orc followers would react if you gave the Orb to one of your followers. I wonder how Cheibriados would react if you tried to make the Orb ponderous. I wonder how Vehumet (once they change him) would react if you killed a monster by throwing the Orb at it. At the very least, all of these should give YAFMs.