This quest starts when you find Ubus standing before the entrance to level 25, talk to him and you will find out that his master went down but hasn't come back. So as usual, venture down into level 25 to save him.

Somewhere on level 25 you will find a room with a beam of light in the middle, and Aeron (Ubus' master) lying wounded beside it. When you talk to him, he will tell you of the evil druids who are trying to block the light from reaching the crystal, and that he had managed to destroy one of the statues before being mortally wounded. He then gives you the Holy Bow, and dies. You don't need the bow to destroy them though, any ranged weapon will do (e.g. magic missile, fireball, throwing axe, bow, etc).

Now go to level 26 and destroy the statue there using a ranged weapon, though if you're using one of the thieves weapons (bow, throwing axe, etc), you will have to hold down shift to hit the statue. Then go to level 27, where you will find another statue, but for this one you have to get past a bunch of teleportation pads, or you can just use the Teleportation spell to teleport over to the other side. Also, one of the teleportation pads will lead to a secret room, this screenshot will show you the exact pad.

The final statue on level 28 will have a group of druids surrounding it, who will attack you as soon as you destroy it. The best way I've found to do this is to stand in the doorway, then shoot straight ahead. If you hit the statue the druids will cry "Kill the infidel" and come rushing towards you, but since you're in the doorway you can just pick them off one at a time.

Finally, back to level 25 where you will be able to remove the Crystal of Strength from its pillar of light.