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EarlyBird The route that Zander suggested is a little problematic:
1: There's a 20 foot drop at the end of the Audience Chamber to the Hydra Lair, which is surrounded in slippery, sucking mud.
2: You'd have to swim, and there's a current that pulls you to the middle of the lake, and eventually around the corner and into a 'bottomless' rift.
3: You can't see it on the map, but the 'shore' that leads to the Pillar of the Sun is quite steep.
4: IIRC a Dracolich Simulacrum collapsed part or all of the entrance to the cavern that the Pillar is in,
AND
5: That's not exactly an Exit shown on the map. It CAN be used as one if a DM chooses, but all information provided suggests (it doesn't outright say, but it suggests, and I went with that one when I described it) that that thing that looks like a tunnel on the map is actually a crack that leads nearly straight UP, to exit somewhere high in the mountain. The crack allows sun to reach the pillar, while the rest of the place is surrounded in black rock. You COULD climb it (and in fact, if I remember right, the Drow prisoner the party let go climbed out that way) but then you'd have to climb back DOWN the mountain.
So... maybe not the best suggestion?