The bestest monster ever

[bighara]

Hmmmm

I've had a lot of luck with throwing Ankhegs at low level parties (to the point where players get nervous when I say the ground shakes). But I love gnolls and ogres for tough humanoids, and trap-riddled goblin lairs.

[Fiffergrund]

I really like Aboleths. I also really like the Aurumvorax, the Revenant, and Crypt Things.

Ropers can be great fun, and the Shrieker is incredibly useful for inducing a manic frenzy of fungi bashing.
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Sir Fiffergrund, Lord Marshal of the Castle and Crusade Society.

He Who Hides Behind The Elephant's Back

[Jungger]

A Gibbering Mouther recently ripped my two bro's PC's a new A-hole. When it steam rolled the wizard, I told him it felt like he was being eaten alive by 1000 piranhas.

[Zebulon]

I had great fun using an alu-demon (AD&D 2e) as a recuring manipulative villain. She wasn't there to slay the PCs, but use them deviously through scheming, deception and manipulation. She was discovered when a cleric PC thought something was amiss, thus did cast a Dispel Magic on her, dispelling her disguise and revealing her true form.

I also had great fun with a NPC thief, master of disguise and scams. Similarly, he tricked the PCs in stealing for him.

Well, I used that sort of trick too often. I always had great fun with it. It usually require a creature (doppleganger, succubus, ogre magi, etc.) able to impersonate other people, and able to teleport at will also helps greatly.
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www.lythia.com: Free resources for Harn but also adaptable to any medieval setting.

[DangerDwarf]

Scariest monster ever is the Type LXIX Demon: The Dreaded Violet Incubus.

Sends parties fleeing in terror every time.