[Radek]

1. Turn Undead has been explained above. Only notes:
Liches are usually level 18 - 27 (Kangaxx the Lich and Kangaxx the Demilich are level 27)
Vampires are usually level 8 - 13
Mummies are usually level 7
Greater Mummies are usually level 14
Your level must be considerably higher than the level of your victim if you want to destroy the undead. Your level must be at least comparable with the level of your victim if you want to scare the undead. Naturally, the Turn Undead ability does not work on non-undeads.

2. Priest of Helm. I don't see any advantage of him with respect to the general priest but I don't see any disadvantage, too. Give your priest a special kit according to his alignment. You cannot spoil anything.

3. The Dancing Sword. You must have the sword prepared but not equipped, i.e., the sword must be in your weapon slot but it must not be in your hand. Now, you get a new icon among your "special abilities" icons. Click the icon. The sword will behave as the Mordekainen Sword: it will go and fight itself without need of your presence or intervention for some time. Then it returns to your inventory.
Notes: The damage caused by the Dancing Sword seems to be higher than the damage caused by the Mordekainen Sword. On the other hand, the Dancing sword is "only" +2. The Mordekainen sword is +4.
You do not get the icon if the sword isn't in the weapon slot of if you keep the sword in your hand.