Here it is! I'm not very good at making homebrew monsters, so all feedback is greatly appreciated!
Hmmm.....for me, seeing as you're going down the construct route, and assuming you have access to the monster manual, I'd suggest splicing the Iron Golem and Hydra together.
For ability scores/AC/HP/Hit Dice and speed use the Iron Golem.
CR will likely need to be a higher but it is largely dependant on the level of the party you were thinking of using it against.
For special abilities: Give it the Iron Golem abilities of Immutable Form and Magical Attacks + the Hydra's Reactive Heads ability. Replace the Golems Fire Absorbtion with the "Unique Immunity" ability
For attacks: replace the Iron Golem's poisonous breath attack with the anti magic blast as the rechargable breath weapon. I'd also think about changing the attacks to Slam or Bite and bring the damage from them down to 2d10 +5 bludgeoning (for the Slam) or piercing damage (for the Bite) + 1d10 Force damage + add something extra such as the slam having a Strength Save or also knocks the target "x" feet away and knocks them prone or the bite having a Dexterity save or the target is grappled in the Hydroids mouth.
****
If you were looking at having some sort of dungeon or mini campaign leading up to the Hydroid then you might also want to take a look at monsters such as Helmed Horrors (CR4), Modrons (CR 1/8 ro 2), Shield Guardians (CR7 but have them bound to a Hydroid Heart instead of an amulet) and Retrievers (CR14), they should give your players a bit of work and ease them into the various abilities they may encounter agianst the Hydroid itself.
EDIT: you could also use Warforged from the Eberron setting as some minions or npc types as well.
https://www.dndbeyond.com/monsters/1838399-hydroid
https://www.dndbeyond.com/monsters/1838444-hydroid-heart
Here it is! I'm not very good at making homebrew monsters, so all feedback is greatly appreciated!
i can roll nat 1s on command
my homebrew thingies
Magic Items - Monsters - Subclasses
Hmmm.....for me, seeing as you're going down the construct route, and assuming you have access to the monster manual, I'd suggest splicing the Iron Golem and Hydra together.
For ability scores/AC/HP/Hit Dice and speed use the Iron Golem.
CR will likely need to be a higher but it is largely dependant on the level of the party you were thinking of using it against.
For special abilities: Give it the Iron Golem abilities of Immutable Form and Magical Attacks + the Hydra's Reactive Heads ability. Replace the Golems Fire Absorbtion with the "Unique Immunity" ability
For attacks: replace the Iron Golem's poisonous breath attack with the anti magic blast as the rechargable breath weapon. I'd also think about changing the attacks to Slam or Bite and bring the damage from them down to 2d10 +5 bludgeoning (for the Slam) or piercing damage (for the Bite) + 1d10 Force damage + add something extra such as the slam having a Strength Save or also knocks the target "x" feet away and knocks them prone or the bite having a Dexterity save or the target is grappled in the Hydroids mouth.
****
If you were looking at having some sort of dungeon or mini campaign leading up to the Hydroid then you might also want to take a look at monsters such as Helmed Horrors (CR4), Modrons (CR 1/8 ro 2), Shield Guardians (CR7 but have them bound to a Hydroid Heart instead of an amulet) and Retrievers (CR14), they should give your players a bit of work and ease them into the various abilities they may encounter agianst the Hydroid itself.
EDIT: you could also use Warforged from the Eberron setting as some minions or npc types as well.