Baldur's Gate II: Enhanced Edition

Enter a locked door in the Bridge District, and defeat the creatures within. The door is near to Delosar's Inn, and requires a Rogue Stone in the party's inventory to open it. Be prepared for the fight beyond it.

For this achievement, make sure you have a Rogue Stone - a very valuable gem that shines in all the colours of the rainbow. The earliest possibility of getting one is probably through Mae'Var's Guild Hall, where you can buy it from Gorch or get one from lockpicing the hardest door in the lockpicking training room.

Once you have a Rogue Stone, go to the Bridge district in Athkatla, and all the way down to the south, and next to the water. You should be between the two southernmost houses by the water. The uppermost one of the two has a door that's facing the water. If you don't have a Rogue Stone in your inventory, opening this door will reveal it to be only a closet. But with a Rogue Stone in your inventory, you'll be teleported to a different place.

Once teleported into the Twisted Rune hideout, prepare for a really hard fight! Your foes will be Shangalar the lich, Shyressa the vampire, Layene the mage, Revenak the thief, and Vaxall the beholder. There will be Time Stops, summoned creatures, and complete mayhem!

Hopefully you'll survive, and then you can pick up the Vaxall's eye from the floor. Put it into the cauldron, then use the machine to return whence you came. The achievement should now have unlocked!