Few things bother me as a DM/GM, but the exchanges below drives me crazy.
DM: A Stone Golem attacks!
Annoying Player: I draw my adamantine mace and attack.
DM: A vampire attacks!
Annoying Player: I grab my holy water sprinkling mace.
I much prefer this version.
DM: A “mnemonic revenant” attacks!
Annoying player: I grab my…Wait, a what? What should I attack it with? Ahh, mommy save me! I do not know how to defeat this new and unique monster.
Nothing terrifies players like a monster they do not know anything about. It really adds a level of difficulty and fear to any encounter. Since most of the iconic monsters in most game systems are well understood by the players, a new beast can really shake them up. Game masters tend to avoid it, since it is more work.
Making a custom, unique monster is annoying in any game system. Most of the time you have to make an educated guess at its stats from other monsters in the rule set. Hopefully, you create a challenging monster without making it an automatic “Total Party Kill” creature. Also, most new monsters do not get a “playtest” with real players, creating the risk of the unforseen.
Such unforseen problem happened to me when I wrote a module with the aforementioned “mnemonic revenants.” They are a type of incorporeal undead that delivers a mental attack. If you succumb to their attack, you have the urge to kill yourself. The game mechanic is simple. You attack yourself with your own weapon and get a “critical hit.” This continues until your fellow PCs restrain you, cast dispel magic or a few rounds pass and the spell fades. I wrote up their stats and ran a mock combat against some PC archetypes, the fighter, wizards, etc. The “suicide” attack worked well and the other PCs had to work to defeat the beast, but it was doable. Since each PC attacked themselves, the high HP fighters took more damage than the low HP wizards, since they each have melee weapons relative to their classes. It scaled very nicely.
Then came the horrible reality. “Living Arcanis” player characters.
The first table I ran had a dwarven PC with a great axe and some feats to amplify his critical damage. He killed himself in two rounds because he did so much damage. Oops. I gave him a mulligan and raised him, but felt terrible. I had not considered a purpose built crit machine in my calculations.
Bottom line: Custom monsters add excitement and an unknown element to any game. Just make sure you know the players you unleash it on and test the thing before it hits the table.
Trask, the Last Tyroomancer