Planar Bestiary
Gloom hazes the air around these flowing, articulating rivulets of "black oil." Like a living creature, wavelets of light-absorbing black slime flow up and down the length of this malign encrustation.
Umbral slimes are spawned in the Shadowfell. A powerful necromancer called Thelicia sought a quick, easy, and "organic" method of using the Shadowfell as a means of planar travel by opening "shadow crossings" where and when she wished, thus creating short-lived portals between the Material Plane and the Shadowfell. She tried to infuse the plane's gloom with an ooze, and succeeded. However, she didn't succeed as completely as she hoped. The only way to create such a portal was to feed a victim to the slime. The corpse itself became the bridge, one lasting only a few minutes.
Thelicia was disappointed but not repulsed. She dealt in death all the time. However, she abandoned her research. Having to procure new victims for each trip—most of whom would prefer not to be so used—was too much trouble. The remaining umbral slimes crawled off into various corners and shadowed niches of the Material Plane and Shadowfell.
Motive: Hungers for life force
Environment: The Shadowfell and worlds of the Material Plane, alone or in pairs
Health: 18
Damage Inflicted: Moderate wound (4 points)
Movement: Immediate; immediate when climbing
Modifications: Speed defense as level 3 due to large size; stealth (in dim and dark areas) as level 6; climbs as level 8
Combat: An umbral slime attacks with a bludgeoning pseudopod of black oil. Each additional successful melee attack after the first requires a successful Might defense task to avoid descending one step on the damage track.
Each time a target adjacent to an umbral slime descends a step on the damage track, some bit of their flesh is converted into black oil, which trickles across to the umbral slime, healing it for 2 points of health.
A target that descends three steps on the damage track dies, and their corpse is subsumed into the Shadowfell over the course of a few minutes. During that period, someone who succeeds on a level 5 magic lore or planar lore task could use the corpse as a portal into—or out of—the Shadowfell.
About once each minute, an umbral slime can cover an adjacent area a short distance in diameter with magical darkness that lasts for about ten minutes. Only magical light can pierce it, assuming the magical light's level is equal to or greater than the level of the darkness.
The slime is an amorphous creature and can move freely even through cracks only an inch or so wide.
Interaction: An umbral slime is more fungus than flesh, and nothing much scares it off other than sustained bright light that it can't quench with its ability to induce magical darkness.
Use: A wizard found the spellbooks and research tomes of a powerful necromancer named Thelicia and is restarting her research into creating portals into the Shadowfell.
GM Intrusion: A character's speech, spell, long drink from their canteen, or similar activity is interrupted when a gobbet of black oil from a nearby (possibly hiding) umbral slime enters their mouth and gets into their lungs, causing them to choke and gag—and do nothing else—until they succeed on a Might task as their action to clear the substance.