Deviser:

Health: 440
Armor: 13

A Golem-like Sentinel with control over thunder. Don't be shocked!

Has a range of attacks, melee and ranged. Can stab and grab you, dealing big damage while you are grabbed, and is able to summon a rain of spears at phase 1. Can apply electrified from most melee hits, increasing damage from lasers, so it is advisable to dodge. Has 2 phases (summons rain at phase 2 as well as able to dodge and uses 2 more devastating attacks). 

Can be summoned (for the meantime) by crafting a Thunderer Summoner block, right click on the placed block to summon the boss. When defeated, drops Thunderium and Thunderous Essence, which are used to create 2 new weapons, Thunder Spear and Deviser Gauntlet, both allowing you to harness some of the boss' attacks.

Boss Music used: Nemesis by Light Canvas.

Video (early) demonstration:

https://www.youtube.com/watch?v=WmydWdD45gU

Scorched Knight:

Health: 270
Armor: 15

Who or what is in that armor? A human? A blaze? Both!? All we know is that it wants to burn all its foes into the ground.....

Consists of melee attacks, a charge, slam, jump, and a special, dangerous move that leaves the boss stunned afterwards. Can apply a DoT called Scorched, which reduces a players offense and defense slightly. (Creative Only as of now)

https://www.youtube.com/watch?v=2pFTXOCyOhY - (Demonstration)

Boss Music used: Dragon Fire by Makai Symphony

The Silent Swordmaster (W.I.P):

Health: 400
Armor: 12

Only phase 1 as of now:
Can dodge & block attacks, can grab and lift you in the air and slam. Has a fire wave attack that can heal the boss when it hurts you, then it can summon small tornados that carry and damage you. Has a dash and a jump attack.

Note: If your system isnt that great, I'd recommend disabling the boss' shadow trail in the config for better performance.