Some Ideas.
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Octoprawn,
Octopus + Prawn.
A boss with the face of a prawn and the legs/tentacles of an octopus. Enraged Plantera + Skeletron Prime AI.
4 Legs behave like Plantera, crawling towards you, While the other 4 Behave simliarily to Prime's Arms.
Has 3 means of attacking.
Slap with a Tentacle.
Shooting ink blobs which inflict slowness and blindness for 4 seconds while dealing damage.
Ramming into you with its main body, Inflicts poison for 20 seconds while dealing damage.
Summoned by fishing with a Vampiric Shrimp (found in water in the underground Crimson)
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Teslashield.
Magnet Sphere + Aegis of ages + 5 soul of Avalon. At a tinkering table AND a superhardmode+ Anvil.
Gives you +300% Defence,no knockback and Cursed debuff for 4 seconds.
Also gives you Mana sickness for 15 seconds.
Costs : 300 mana.
Cooldown : 1 second.
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Crystal Blast.
Inferno Fork + 50 Crystal Shards + 10 Souls of Avalon. At a tinkering table AND a bookshelf.
Spawns a ''turret'' Much like the Nimbus Rod.
This turret sends out Crystal Shards in random directions continously. Roughly 10 each second. (like the ones which spawn when a crystal shot hits an object, they despawn after half a second or so.)
Each Shard deals 30 dmg.
Turret stays stationary for 1 minute or until another Turret type summon is placed.
Costs : 120 mana
Cooldown : 1 second.
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Parasitic Swarm.
Generic Contagion Material X50 + 5 Soul Might + 30 Ectoplasm. at a demon altar (or equivalent)
Spawns a Parasitic Swarm with the AI of a Dungeon Flame Wheel. the swarm is NOT a summon, but a Magical projectile.
Lasts 4 seconds
Costs : 60 Mana
Cooldown : 1 second.
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Stellar Torch.
5 Soul of might + 5 Soul of light + 10 living fire block, at a demon altar (or equivalent)
A place able light source Which lights nearby Hostile mobs on fire.
Think
Inferno Potion effect, but in a block. no fire ring tough.
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Anyone with a decent head on their shoulders is welcome to edit/expand these ideas.
as they are only that, ideas. hopefully one of these ideas has sparked some others imagination.