I'm running the first off-world adventure for the recurring cast in my AD&D 1st Ed. campaign. It is a holy quest for the fabled Lonsdale Belt, an Asgardian artifact made by the dwarves of Nidavellir, stolen by a Type V Demoness and taken back to her stronghold on the 4th plane of Tarterus.
The party, led by a priest of Odin, have to travel across the Astral Plane to Tarterus, then find their way through the first three layers, going via portals between the levels, to the fourth plane, where the demon's fortress lies within the realm of Grolantor, god of the hill giants.
This adventure is quite a departure for the party, who are around 10th level. My rule tweaks for adventures on other planes have been quite a shock to the system, as I've changed things so that the PCs remain unchanged while everything around them changes. Basically, anything native to the outer planes has its hit dice and damage doubled. The effect is to make these powerful characters rather more nervous than they started out. Complacency is a dangerous thing in this setting!
They are currently 9 sessions in and are still on the first plane of Tarterus, having suffered an accidental petrification requiring them to do a deal with a local magical guild called The Cabal to perform a task in return for a cure. That task turned out to be dealing with a local protection racket run by a malebranche with a squad of barbed devil enforcers. They have just succeeded in defeating the devils following a climactic confrontation in the bad guys' base. I believe they have learned some valuable lessons about the scale of things in Tarterus, that should serve them well for the rest of the adventure.
Most awesome moment so far was last session, during the climax, when things were going decidedly badly for the party. The head priest let rip with his Sunray (1/day effect of a magic item) which damaged and blinded the malebranche and two of his remaining heavies (and blew up his imp familiar), but also produced enough natural sunlight to allow the party witch to cast her Laser Beam spell (only castable in natural sunlight), which did double damage on undead and fiends and in this case did 66 HP damage to the malebranche and was the turning point in the fight.
"...with frikin' lasers!"