Sometimes it's nice to shake the players up a bit. They know the game a little too well and are becoming hard to surprise. One trick is to look at your bookshelf and steal something from a very different game and stick it in the one you are running now.
As an example, I'll steal a Numenera creature, and 13th Ageify it. I'll take one of mine, the Pyronic Sentry, which first appeared in The Sun Below: City on the Edge, but you can see it here. You can do this with any two game systems.
This is a challenging creature for beginning Numenera parties, so let's make it a challenge for a 2nd level 13th Age party. We'll make it a large 4th level monster. It looks large and it's doing double damage, which is what large monsters do.
It's obviously a construct, perhaps one of the Dwarf King's artificers went over the deep end and sold these to the highest bidder. Or was forced to make them against her will...
Reading the modifications, we'll make it invulnerable to heat, and vulnerable to cold.
Now, let's look at it's GM Intrusion: it can teleport a PC to directly in front of it. Which is nice, because it's always standing in a fire or lava, and then it hits it for extra damage. A GM Intrusion typically only happens once a battle, so we'll go with limited use, once a battle.
Check out DIY Monsters section in the 13th Age Core book.
A pyronic sentry is an armored construct whose color changes to match the flames it finds itself in. It looks like an eight foot tall armored man with long blades for hands. It is always found standing in fire, lava, or some other superheated environment. To engage a pyronic sentry, one must enter its fiery environ. As a construct it follows orders to guard a place or a thing.
Large 4th level wrecker [construct]
Invulnerable: magical and mundane heat and fire; Vulnerable: cold
Sizzling Sword Hands +9 vs. AC -- 20 damage
Natural even hit: +16 heat damage
R: Come Play: +9 vs. MD -- 24 damage
Hit or Miss: Nearby target is teleported directly in front of the pyronic sentry, taking 16 heat damage from the environment the sentry is standing in.
Limited Use: 1/battle.
Fiery environs: At the start of the pyronic sentry's turn, each enemy engaged with it or otherwise standing in the fire/lava takes 14 fire damage.
Some Like it Hot: A character must make a hard save (16+) to leave the fiery environs.
PD 18 HP 108