After a duergar theurge called Framath helped the PCs in
their fight with a hydra, which
left the PCs in possession of a 4th piece of the Sceptre of Law (= Rod of 7 Parts), the PCs returned with her to the duergar stronghold so that they (i) take an extended rest, and (ii) repay the debt they owed her for her help. (They made it clear that they were
not going to give her even a piece of the Sceptre as repayment!)
In the duergar stronghold, the PCs were housed in two separate groups. The invoker/wizard who wields the Sceptre, and the tiefling paladin who respects the duergars' reliability and empathises with their past dealings with devils, were housed in pleasant quarters. The dwarf fighter/cleric of Moradin, the elven ranger, and the drow chaos sorcerer, were housed in dingy quarters. The drow was in a bad way for two reasons: (i) his channelling of chaos energy in the fight with the hydra had blinded him
and has Robe of Eyes (although he was able to attune himself to chaos emanations, gaining a variable blindsight); and (ii) the duergar didn't trust him, given that he is a drow wearing demonskins emblazoned with runes of chaos.
<snip>
the drow received a mysterious telepathic communication from a being calling itself "Pazrael", telling him that if only he would open a doorway in a warehouse in an out-of-the-way part of the hold then his blindness would be cured. Deciding that he was sick of being cooped up, the drow sneaked out to the designated location, and after some scouting out, and some wrestling with his conscience, decided to "open" the door by blasting it with the Winds of Change. When he did (i) his blindness was lifted, as promised, (ii) a magical gate opened in the doorway, and (iii) some duergar guards heard the (loud) noise and came to investigate. They found the gate, but the drow managed to persuade them that he was in the area because he heard the noise, rather than that he was the one who had helped open it. (This was resolved as a skill challenge, with only partial success.)
The guards nevertheless took the drow before the leader of the hold (and Pechuk's rival), Murkelmor.
<snip>
news came to Murelmor that hordes of demons were now attacking through the gate. Naturally, the PCs were in the vanguard of the response! This fight unfolded in two stages. First, the PCs (minus the ranger, whose player had to go home - we took it that he was on his flying carpet helping the duergar off-screen) came to the gate and fought the demons guarding it - a nabassu, a shaadee and a (levelled up) Jovoc - as well as a flight of vrocks that flew in to attack them (elite Gargantuan swarm). The two biggest tactical challenges here were (i) bringing the vrocks to ground - once they did this, they didn't have too much trouble with them), and (ii) the shaadee's domination, which I got off twice in a row against the sorcerer PC, who was therefore blasting away the defenders' hit points with Blazing Starfalls. Once the sorcerer's player got his actions back, he blasted the demon away with some well-placed demonsoul bolts (which pushed it off the roof of the building it was lurking on, for bonus damage). But just as the PCs were finishing off the demons, they got a shock when Orcus (or, as it turned out, an Aspect thereof) came through the gate.
At the same time that this happened, the ranger PC came onto the scene chasing mobs of orcs (two Huge swarms), plus half-a-dozen ogres - slaves of the duergar in revolt under the leadership of an ogre dreadnought and trying to escape through the gate. (In the real world, Orcus's arrrival marked the end of one session, and when we reconvened a fortnight later the ranger's player was back on deck.) The players opted to focus on Orcus while holding off the slaves, or picking them up in AoEs. The paladin was maintaining a Righteous Inferno, which in conjunction with some other terrain features forced the orcs and ogres to focus on the defenders rather than the squishy sorcerer and invoker.
As his 19th level daily the invoker's player had chosen Forced Submission (a relibale save-ends domination power), and decided that now was the time to use it. He hit first time, despite needing 15+ - he had bonuses from Preserver's Rebuke, Knowledge is Power (Divine Philosopher ability) and combat advantage due to Orcus being in the paladin's Inferno. So Orcus spent two turns dominated: on the first he threw his Wand through the gate, which the sorcerer then blocked off by raising a pillar of earth in front of it (6th level utility from Heroes of the Elemental Chaos); on the second he clawed ineffectually at the ogre leader (who had got too close to the invoker for the invoker's comfort!).
After that it was all downhill for Orcus, as the ranger pelted him with arrows, and the other PCs wailed on him too), and while the revolting slaves managed to pile quite a bit of damage onto the defenders (especially the fighter), good healing tactics kept them up. Meanwhile the invoker shut down the gate - this took four actions (three standards and a minor) but was done in a single turn using a Timeless Locket and Uncanny Insight. In the course of this, he worked out that the gate had runes of Pazrael inscribed around its edge, but that it was not opening onto the Plane of 1000 Portals. He also detected a strange disembodied presence, which offered him help in shutting down the gate. He declined the help, but opted to shut the gate quickly rather than try and force the presence back through it (mechanically, he finished the skill challenge at 4 rather than 6 successes). Shutting the gate hurt Orcus further, debuffing him on defences and damage, and he was killed (fittingly enough) by an arrow from the ranger, who is also a cleric of the Raven Queen. (The paladin, who also serves the Raven Queen, and whose turn was up next, was forgiving of this kill steal by a fellow devotee.)
At this point it seemed like things were OK. The PCs had seen Murkelmor fall in trying to quell the slaves, but otherwise things looked OK. When the disembodied voice vowed vengeance against the invoker and his duergar friends, he laughed it off and set about conjuring a Magic Circle to trap the body of Orcus's Aspect, and thereby (hopefully) depriving Orcus of some of his essence. (He used a jewel carved in the shape of an eye that had earlier been taken from a worshipper of Vecna as a focus to make the Circle permanent - the players liked the idea of having Vecna be their watcher over Orcus.)