Check your conscience before you go to bed. Check under your bed, and in your closet as well, especially if your conscience isn’t clear. And I mean really clear, not just rationalized into something you think qualifies as clear. The sudden feaster smells your sins, and their stench whets its appetite for flesh. Your flesh. When a sudden feaster appears, seemingly out of nowhere, its fanged maw gapes wide and snaps shut fast and hard enough to sever limbs, making it all the easier for the feaster to gorge itself on your blood.
A sudden feaster is a fearsome, ogre-like creature driven by dark urges to hunt and devour the wicked. This monster is not, however, some righteous avenger of wrongs. Rather, it delights in the pain it inflicts, in the seeds of terror it sows in its victim’s life before it strikes.
Stats for Barbarians of Lemuria:
Attack: bite +0 (damage d6+2)
Protection: d3-1 (magical hide)
A sudden feaster can appear from and disappear into spaces too small to accommodate its bulk, doing one or the other once per round. If its victim cannot see it, a sudden feaster may make a check, modified by its mind and opposed by its victim’s mind, to create a sense of dread in the victim.
Stats for Dungeon World:
Intelligent, Large, Solitary, Stealthy, Terrifying
Bite (d10+1 damage, 1 piercing)
Close, Forceful, Reach
Instinct: To eat the wicked
* Appear from or disappear into surprising places
* Make scary noises
* Sniff out the wicked
Stats for Swords & Wizardry:
Hit Dice: 6+2
Armor Class: 5 
Attacks: 1 bite (1d10+1)
Saving Throw: 11
Special: appear/disappear, sow terror
Number Encountered: 1
Challenge Level/XP: 8/800
A sudden feaster can disappear into or appear from spaces too small to accommodate its bulk, doing so once per round. If it is unseen, it can sow terror by scraping its nails slightly on a solid surface, causing a board to creak ever so lightly, rapping on a window pane, et cetera. The victim’s idea that something might be lurking nearby festers in the subconscious. The victim must make a saving throw or suffer a fear-induced –2 penalty on attack rolls and saving throws for 1d4 rounds.