Hey! Glad to see another new DM getting into the hobby. I would suggesting two things:
- TAGs - cool, evocative words that remind you what the creature is and how it works
- STATs - based off a die (or dice); not how everyone does it, but I like it. You can also do as Hardcore suggests and use a CR.
TAGs
SWARM
DRAIN, CLING, QUEEN
The Swarm are dangerous to even the most experienced. Latching on, they slowly drain the STATs of players. Find a way to influence Syrah’s state of mind or exterminate the swarm of fist-sized insects before they can suck the life from you.
Eel-men
SLOW, PUTRID, SLIMY
Eel and man combined. They splash through the muck after prey.
STATs
STATs are simple! Dont worry about balance. You will never get it right trust me. You can always dial it up or down. Pick one or several dice to represent the creature. Write it down and move on to thinking about funny voices, evocative art, or other cool shit for the session.
If things are too easy, an Eel-man spawns with a particularly long neck that can strike from a distance or wrap prey. See this video on challenge tuning.
SWARM
I think a d4 sounds good. I will base the its STATs and actions off that
HP: - (the challenge of the swarm is calming down Syrah; they are infinite)
ATTACK: +2 (d4*1/2)
DRAIN: reduce 1 stat by 1 at random
CLING: every round in a SWARM take d4 damage automatically
Put the players in a corner and let them figure it out!