Stalker

Humans that can quickly go invisible. They sneak up on intruders and try to kill them quickly.

A stalker attacks in melee range. A stalker never retreats.

You can't attack this creature with melee while it is invisible. The only way to defeat them is with Magic or by making them visible again by attacking them with area runes (Avalanche, Explosion, GFB, Stone Shower or Thunderstorm), making them walk over a trap, an area spell like Berserk or using Cancel Invisibility. Sometimes they don't go invisible right away or at all and can be meleed. If you can't do anything but melee you can take stalkers by going down/up stairs or a cliff and waiting until they are visible, then attacking them.
Knights: Make them visible using one of the runes above or use Berserk, Groundshaker or Fierce Berserk, then attack them with melee. They are weak in defense, so a knight can easily kill one. The will also work to kill stalkers, even if they are invisible.
Paladins: Make them visible using one of the runes above or use Divine Missile or Divine Caldera and attack with distance.
Mages: Make them visible using one of the runes above or an attack spell. Another good idea, since stalkers always stick by you, is to hotkey one of the runes above and set the target as yourself. Since it doesn't harm you. Keep an eye on your hp bar, as it is hard to tell how many stalkers you are actually facing. If there is a large amount, your hp could go from full to empty without you noticing.