Here are a couple of ideas for fiend enemies. The first one has been posted before, but the second is brand new.

Bishop
Just as Trojans are corrupted knights, Bishops are corrupted snipes. They are called Bishops because the purple crystal on their head, wider beak, and stony appearance make them resemble the top of a chess bishop. They are larger than normal snipes, being almost the same size as a knight. They have mouths on their wings. Lower tier Bishops are darker than upper tier ones, and tier 3 Bishops have the same flaming, purple eyes that tier 3 Trojans have.

They have three attacks:
Flying - When moving around, Bishops fly just like snipes do. When flying, their wings create a razor-sharp circle around the Bishop that harms any knight who touches it. The Bishop is immune to all attacks while flying.

Lightning Storm - The Bishop puts its wings (or just one wing if there is only one knight) into portals. Each wing then appears out of another portal over a knight's head. After a split-second (or so) delay, each wing will start shooting continuous purple lightning out of its mouth while slowly following the knight it's targeting. In tier 1, the wings shoot only one bolt of continuous lightning. In tier 2, the wings shoot a tightly packed group of bolts. In tier 3, the wings shoot a group of bolts with a wide spread. In all three tiers, this move can cause shock and will damage other enemies as well as knights.

Lightning Ropes - The Bishop creates a purple "lightning rope" (similar to the "fire rope" in Firestorm Citadel) out of each of its wing mouths, then rotates its wings around itself. In lower tiers, the lightning ropes get longer and the Bishop rotates its wings faster. In tier 2 and lower, the Bishop can walk around slowly while the lightning ropes are out. In all three tiers, this move can cause shock.

Just like Trojans, Bishops don't have status variants. Admittedly, I don't know how or why a snipe would get corrupted the same way knights did. I mostly just did this for fun.

Other Thoughts:

• The Bishop doesn't have to be a corrupted snipe. If the corrupted snipe angle doesn't make any sense, the Bishop could be a bird-like fiend without any connection to snipes at all. In this case, their flying attack will be altered and they will have status variants.
• The wing mouths aren't necessary for the design. I just added those to make the Bishop more fiendish.

Anglock
Anglocks resemble immobile, purple deep-sea angler fish, except its "lure" is a large ring that it places in front of its mouth. Lower tier Anglocks have spikes on their bodies; tier 2 Anglocks have a few small spikes, while tier 3 Anglocks have many large spikes. They are the same size as other turrets.

As Anglocks are turrets, they only have one attack:
Turret Fire - The Anglock shoots a bullet. Immediately as it does so, it creates two portals; one portal is in its ring, and the other is somewhere around a knight (and aimed toward that knight). Once the bullet enters the Anglock's ring portal, it will reemerge out of the other portal. Bullets from tier 2 Anglocks will split into three bullets upon reemerging, and bullets from tier 3 Anglocks will split into five bullets upon reemerging. Both portals disappear as soon as the bullet reemerges (or if it disappears before entering the ring portal). Neither portal does damage on its own, and the bullet can damage knights before entering the ring portal. Anglocks have the same rate of fire as polyps. Anglocks will turn and face a knight before firing.

Status Variants:
Fire: Fire Anglocks are called Charlocks. They are red, and any spikes they have are on fire.
Freeze: Freeze Anglocks are called Iciclocks. They are blue, and they have icicles instead of spikes.
Poison: Poison Anglocks are called Plaguelocks. They are green, and they have snake fangs instead of spikes.
Shock: Shock Anglocks are called Volocks. They are yellow, and any spikes they have are electrified.