Kharos used to be one of the hardest place that exists in the game, besides the Pits of Inferno.
Those with low level should be careful as they might die instantly when entering.
This tower is filled with all sorts of terrible creatures, including many Liches, Serpent Spawns, Warlocks, Dragon Lords, Behemoths and Demons.
The tower was sealed off by the Edron magic academy. To access the tower, talk to NPC Zoltan in Edron and ask for "permission", You must step on the energy wall for Zoltan to acknowledge the existence of the citadel and grant you permission. He will let you enter by paying 500 gps (note that if you paid once and entered there, you have to pay again). To check if your character currently has permission, open your quest log, look for "The Shattered Isles" and then "Access to Ferumbras' Citadel". If you have access, you'll have the entry, and it will be marked as "completed".
Warning! When you step into the citadel, you will be greeted by several behemoths, and possibly lured demons and dragon lords. If you are low level (lower than about 150), you are advised to not move at all until everything around you is dead. Stay at the wall so "only" 5 creatures can reach you. If you are high level, you can run north-east to the basement stairs.