"A human-sized blob with hanging tendrils (5)."
Done. The GM knows it's a difficulty 5 to dodge the blob, hit the blob, or if the blob is intelligent, sway the blob to the PC's point of view. If the blob gets mad and hits someone, they take 5 points of damage. The blob has 5 * 3 = 15 hit points. All from the number 5.
But take a look at the creatures in the core book or the Ninth World Bestiary. It's OK to add all sorts of individual quirks to a creature. Best of all, you can add a creature specific GM Intrusion, just like in the books.
When I'm designing new creatures for an adventure like The Sun Below: City on the Edge, I always add a GM Intrusion. But why? Why would you use one in game?
Ever forget to give every player a GM Intrusion? Forget no more. If you have a combat, open with one. That gets the players' attention and signals the situation is going to be intense. Have 4 players, and you think you'll be running 2 combats this session? Throw 2 GM Intrusions into each combat. Ta-da! You've met your quota.
And every time your players roll a 1, you know what to do.
Of course there are many non-combat GM intrusions that are just as fun, but I'll talk about those another time.
Here are my rules of thumb when designing a creature GM Intrusion. These are just mine. You will have your own, but you know, sharing is caring.
- In other games, creatures often get special powers that they use only sometimes. A dragon's breath, extra damage if they roll certain numbers, when they reach half health, and so on. A GM Intrusion is the Numeneraish way to give those powers.
- Make it flow from the creature's normal abilities. Got a creature with psychic powers? They can use Mind Control as a GM Intrusion. Are they giant size? They can stomp on a PC's head.
- Make it powerful, but tied to the creatures's level. A level 3 creature probably should probably not pop a death ray that drops a character 2 steps down the damage track. I often go with either an extra attack at 2+ levels over the creatures level for a ton of damage, or an automatic hit for half a ton.
- You don't have to "hit" to have an effect. If the player spends a lot of effort to avoid the effect, they are down points, and deserve the benefit.
- As your players go up tiers, they get free effort from edge, so don't be shy about bumping the difficulty to avoid the effect.
- If it's a high level creature to begin with, it's great to have a GM Intrusion that drops a character down a step on the damage track. Other fun effects are dazed, stunned, and knocked off a cliff. Into a sea of lava. With spikes.
- I stick to single target effects. Why? If I want to pass out GM Intrusion XP, single target keeps it simple. Yes, if the player rolls a 1 it might be fun to pop a big area of effect hurt on the players, but creature powers don't lend themselves well to group GM Intrusions for XP.
- I like to add a line I can use to describe how it feels.
You teleport into the flames and feel the searing hot bladed hands of the automaton dig deep into your body. Your flesh sizzles like meat on a spit.