Captain Ahab's Leg posted a wonderful evocative system for generating unusual encounters based on Tarot cards. I love random content generation, I have a Tarot deck, so... here we go!
The Form: Eight of Pentacles
A craftsman chisels pentacles on golden discs – perhaps these creatures (because there are several of them) are constructs.
The Human: The Fool
Perhaps these monsters can be underestimated at first…
The Inhuman: Death
…but then they present a significant threat. The image shows Death as a mounted knight. A high priest in a tiara stands on the ground before the knight. Maybe praising him, but maybe trying to barter with him. In any case, the knight towers above him, subdues him just as he’d subdued an enemy previously. So perhaps the creatures have some sort of awe-inducing mind control power.
The Larva: Six of Wands
The wands are carried, not mobile on their own. In their initial state, the creatures need an outer force to propel them forward. Maybe tumbleweed? They roll in, surround the enemy, and then build up into something unified and bigger.
The Pupa: Queen of Swords
Majestic and bellicose… But the Queen sits upon a throne, doesn’t fight. She towers over the enemy, and prepares to unleash the final attack.
The Imago: Nine of Pentacles
Pentacles again, but at a higher value than the Form, so I interpret this as a growth in power. The image shows a falconer, so maybe the great monstrous attack (the “awe-inducing mind control”, see above) is a swift, targeted thing.
First Encounter (Eight of Pentacles, The Fool, Death)
An abandoned workshop, eerily silent, dead bodies lying around it – no evident cause of death. Inside lie eight golden discs with pentacles engraved on them. A priest kneels in the middle of the room, mumbling “please don’t hurt me, oh most magnificent one… please don’t hurt me!”
Second Encounter (Six of Wands, Queen of Wands, The High Priest)
The priest suddenly snaps out of the condition and starts arranging the discs in a specific shape.
Third Encounter (Nine of Pentacles, Ten of Wands, The Judgement)
A force of magnetism starts pulling together the discs, they gather, lift up from the ground, and form a sphere mid-air. They shine gold and bright, majestic… And now one person of the party is struck with awe – they must serve the sphere.