Summary
It’s always fun to test new monsters out with your players, terrorizing them and making them think they’ll lose their characters when, in reality, you’re hoping you’re not going too hard on them, right?Dungeons & Dragons' Quests From The Infinite Staircase adapted a few classic adventures to fifth addition, and with them, new creatures to play around with.
You have quite a few exciting picks, from aggressive plants to powerful bosses and even robots, and some can be powerful allies to have - monsters in D&D are just a fancy way to address NPCs, after all.

10Android
With Many Variations
We are used to automatons, constructs, and golems, but robots are not something you often see in D&D. The androids are mostly just to punch the players and maybe leave them prone with said attacks, but what’s fun about them are the specific characteristics they may have depending on the variation.
These androids have six variations: aerialist, diver, medic, duelist, diplomat, and sentry. you’re able to pick whichever fits the situation or roll one at random, and they give the machine a different ability, such as flying speed for aerialists or healing spells for medics.

9Combat Robot
EXTERMINATE
These robots are more straightforward, without any sort of variety, but what makes them shine is advanced firepower. They can grapple and restrain up to two creatures at a time with their tentacles, they have access to a hard hitting laser beam as their default weapon, and they can launch grenades.
That along with their high health pool - which can be countered if the players have already foundthe futuristic equipmentin the adventure - makes them quite dangerous, especially in high numbers.

8Leprechaun
For Good Luck
As monsters go, the Leprechauns aren’t particularly dangerous. Their tricks mainly consist of annoying the players or making things, as well as being, lucky. Still, they can cause damage, steal things with a bonus action, and share their luck with allies - which can be the players.
They also have a fun mechanic where they may get charmed by someone through music. All in all, they’re not the best pick for a big fight, but you’re able to use their stats for unique interactions with the group.

7Horrid Plant
Also With Variations
Another example of not being a big fight but still interesting is these plants - though they can still fight and are more dangerous. They come with variants, just like Androids, and they can catch players by surprise by being motionless. They are plants, after all.
Some can cause acid damage, others can hit everyone around them, and some can use life-stealing attacks. And all of them can grapple enemies. Mix some of these together to start a surprise fight because no one was expecting the plants to attack, and you’ll have a fun fight here.

6Wolf-In-Sheep’s-Clothing
More Tricky Plants
The idea of how to use this plant is similar, but the Wolf-In-Sheep’s-Clothing is stronger, having more abilities to use - though you should still have more than one in a fight, as they can still be taken down easily by themselves.
The fun part here is their main gimmick, where they can create a false tiny creature or small object to attract the attention of their victims. So you can distract your players with a fun little creature before going on the offensive.

5Memory Web
For Combat And Storytelling
While still on weaker creatures with a unique design to it, the Memory Web has the power to grapple a target and slowly drain their memories away. Draining memories from the target also makes them perform worse on their ability checks, so it can be quite dangerous if you stay grappled for too long, and it can be troublesome to recover from it, with spells likeGreater Restorationbeing necessary.
Lastly, destroying it causes a powerful memory burst, spreading drained memories to everyone near the creature, allowing some cool roleplaying opportunities to start after the fight.

4Froghemoth Elder
Ribbit And Rip It
Now, let’s focus on the big monsters you’re able to find here. Froghemoth is huge, has strong attacks with a lot of reach, can jump around and hit people with its gaze,can deafen and frighten,and swallow people whole.
Overall, the big frog is just an interesting monster the party will have to deal with during the adventure, and it’s perfect for a high-level group to fight against. And he’s even got a bit more features than we mentioned.

3The Gardener
And He Doesn’t Kill The Players
The sad part about the Gardener, combat-wise, is that he has zero interest in fighting the party - he’s even a bit overpowered compared to the players' levels, as he’s not supposed to be an enemy. Still, he has quite a few fun tricks if you want to make him violent or homebrew a new archfey based on his stats.
He can help people resist spells, use tricks that make targets go unconscious without having to fight for long, grapples, and spells of his own, not to mention the whole garden he has, with lair actions included.

2Zargon The Returner
Somehow, Zargon Returned
Zargon is also a creature you may want to save, as he appears in an adventure that goes from levels one to four while having a CR of 17. The adventure offers some tips to expand it, however, and you can easily turn Zargon into the BBEG of your campaign.
He causes massive damage and has plenty of features that stop or prevent spellcasting for a while, and killing him is a quest in itself since destroying his body isn’t enough to take him down. Returning is his thing, after all.

1Nafas
Your Quest-Giver
This is also one you may want to homebrew if you want him to fight, since he’s the one who even recruits you for the missions here. That said, Nafas is a noble genie, and his CR is above 20, meaning he’s very powerful.
The fact he can cast Wish alone is very powerful, but he has spells, can cause a lot of damage and cause conditions at the same time, and has powerful lair actions. You could have quick fights against him as training sequences with the party, where Nafas wouldn’t fight with everything he’s got, depending on their levels.