Leatherhead
Possibly a Idiot.
Astral Dreadnought (MToF)
The Astral Dreadnought is a CR 21 flying eel with crab claws and a beholder’s antimagic eye. Unsurprisingly, they are encountered on the Astral Plane, and can’t leave the place even if they want to. Their only purpose, it seems, is to hunt and kill mortals who use the Astral Plane as a means of travel. For this they have two powers, the ability to sever Silver Cords on a critical hit, which instantly kills, and a rather unique ability to shove anything they eat into a pocket demiplane (which also causes the astral cord to reel the character’s physical body into the demiplane from the material world). Which means this creature belongs in a category of monsters designed just to give the screws to the players, much like rust monsters.
All that out of the way, there are few interesting things you can do with a Dreadnought, it’s personal donjon is essentially a mobile treasure vault, and any number of important things or people could be trapped inside. If you manage to kill one, the donjon disappears, and everything it ate suddenly appears in the astral void around it’s corpse. Which could leave a character who was previously trapped in the donjon, now trapped in the Astral with no quick way home. This being trapped in the Astral problem could even occur for someone who gets hit with the Donjon Visit power, meaning even if you win you could still suffer greatly from the encounter!
Before I get into combat, I have an important question for you all to consider: You are under the effects of an Astral Projection spell, speeding along, and suddenly you hit an Antimagic Field, what happens?
According to AMF:
“Magical Travel. Teleportation and planar travel fail to work in the sphere, whether the sphere is the destination or the departure point for such magical travel. A portal to another location, world, or plane of existence, as well as an opening to an extradimensional space such as that created by the rope trick spell, temporarily closes while in the sphere.”
And for Astral Projection:
“The spell might also end early for you or one of your companions. A successful dispel magic spell used against an astral or physical body ends the spell for that creature. If a creature's original body or its astral form drops to 0 hit points, the spell ends for that creature. If the spell ends and the silver cord is intact, the cord pulls the creature's astral form back to its body, ending its state of suspended animation.”
Now a DM could argue that the Antimagic Cone power causes an Astral Projection to wink out of existence temporarily, like a summoned creature. They could also argue that it just ends the spell. Or they could even argue that the AMC means those trapped inside won’t automatically bounce back to their real bodies when they hit zero HP. Personally, I think the last option is the best one, as it actually enables the creature to do what it’s intended to do.
Assuming my interpretation, the Dreadnought is a big dumb brute who just swims up to one of the PCs and tries to eat them like a shark. Using it’s bulk, speed, and Antimagic Cone to focus on what it perceives to be the easiest target out of the pack. Chopping them down as fast as possible, and if necessary speeding away from the survivors. It’s very fast for its size, but does not have much in the way of tricks other than raw DPR. Meaning the good old hit and run tactic is it’s best bet. Which could be an interesting plot hook as the remaining PCs now have to track down this apex predator. The Dreadnought has some other options in combat, but honestly they aren’t the best because of how many actions they cost. The Donjon Visit power is a temporary control that can force a target totally into the Astral for later consumption, or provide a brief reprieve from a nasty target that's close by. Psychic Projection is an AoE option, but rather costly at three legendary actions, and honestly it might be better to use those actions to just claw a single target down.
Interestingly enough, the only changes to the Dreadnought are the removal of the Magic Weapons trait in favor of Force damage on it’s melee attacks, the addition of Unusual Nature, some language cleanup, and their Psychic Projection getting more damage.