Just give us a punching bag and the ability to name it anything and everybody will be satisfied.

Well, except for the punching bag.

If anyone remembers what slags used to be like before the nerf, pretty much that, fast, hard to dodge, and dangerous in groups

A corrupt knight.

Randomly equipped from 0-5* depending on the tier, and using the weapons equipped to fight. Possibly using a pet too. Can shield, dash, shield bash, charge attacks and everything else, except he's all VH. His defense corresponds to his currently equipped armor and helmet. Spawns as a random story room boss.
Kill it just like you would kill another knight.

It would be kind of neat to have a sort of running boss with a unique twist - players are forced to use the striker boost from lockdown (does not have damage/ASI boosts or CTR penalty, and also have infinite boost).

Why you ask?

Because between phases of attacking the boss, it would roll up into a large rocky golem and roll away from you on a circular track. You would need to use your striker boost to pursue it, as staying at the area you were fighting in before would crumble and hot gasses would emit from beneath, harming your knight. This is the case with the track the golem escapes on, so you need to stay close behind while it rolls away - however its movements rock the ceiling, dropping hazards in your way!

Setting for this is inside a dormant volcano that has recently shown unnatural signs of activity. Due to its close proximity to research camps, a squad of knights were sent to investigate, but haven't returned. We are sent in to discover their fate and continue what they started.

So let's get started with how it all works now shall we?

Levels leading up to this boss would be fire themed while being a mix of slime (rock jelly, blast jellies), beasts (fire dust bunnies, alpha wolvers), and fiends (fire devilites, fire overtimers, fire gorgos, fire greavers, silkwings). But those are only a setting.

After venturing through this volcano, you end up in a room next to the main event. Activity is all around. Magma is boiling everywhere, but the original squad is holding out in this room. They give you information about what they've discovered, but also that something horrible and very powerful is up ahead. It crushed their squad's guardian in one fell swoop, their squad leader had been burnt to a crisp by the hot steam from under the rocks, and the rest of the squad fled in fear, watching helplessly from this side room. However, a few crafty technicians have been working on upgrading a few striker shields they brought along for when they go in for round two, but seeing your squad arrive, offer them to you.

In this room there is an arsenal station and a monster codex.

Rock Golem
This boss appears to be something summoned by fiends, and is embedded with the spirit of a tortodrone. Heck, it might as well be one of the tortodrones you've defeated that the fiends took back and attempted to make stronger.
This boss has moderate resistance to all damage types except for normal, and is immune to fire, poison, and shock. It has a high resistance to to curse, and has a slight weakness to freeze (however it is immune to minor freeze).
This boss has three phases, and many attacks.

Attacks
Phase 1
• Golem Smash - The Rock Golem smashes the ground in front of them, causing rubble to fall from the ceiling. The smash itself deals high damage and stun, and the falling rubble can inflict fire upon hitting the ground, but touching the rock itself afterwards is harmless. You can break rocks by hitting them 3 times.
• Golem Punch - The Rock Golem swings a fist forwards, lunging forwards and dealing high damage to whoever it hits.
• Summon - The Rock Golem drops to the ground for a moment, and 2 blast jellies and (phase *2) rock jellies wiggle out. In later waves, fire gorgos also emerge (one gorgo per phase). Cannot summon while 2 summoned monsters remain.

Phase 2 (retains all attacks from phase 1)

• Ricochet - Rock Golem rolls into a ball (same mode it uses to escape), and bounces around the room, bouncing off edges and walls. It will also break any rubble it comes in contact with, so you can't use it as cover from this attack!
• Rock Sprinkler - Rock Golem burrows slightly into the ground, then spins into the earth, uprooting rubble and firing it off at high velocities. These rocks will moving in a line circling the golem - it's about 50% as fast as the striker boost in this phase, and 80% as fast in the final phase!

Phase 3 (retains all attacks from phases 1 and 2)

• Shockwave - Rock Golem smashes the ground, causing a powerful shockwave to expand, stunning any player it hits. It phases in and out in 2-3 tile intervals, meaning for 2 tiles it's hazardous, and then for 3 tiles it has no effect, then the next 2 tiles it will be hazardous, etc.
• Earthquake - Rock Golem shakes the earth, causing a lot of rubble to fall from the ceiling and also opening up pores on the ground which erupt with hot steam that can inflict fire (however it cures stun). These pores are now forever open unless falling rubble from another golem attack lands on these pores, and seal them. Breaking these rocks after they've sealed the pores will not open the pores back up.
• Portal creation - Rock Golem uses fiendish energy to summon fiend portals, which act exactly like they do in the March of the Tortodrones event. However these portals can be entirely destroyed and do not respawn until the golem summons them again.
• Final Breath - Rock golem, after having all health depleted, falls to the ground, shaking the earth. Doing so makes magma pour onto the arena, and two large vog cubs show up! They act like the snarbolax but without the armor plating, and inflict fire.

Alright, with attacks out of the way, let's move on to the new part - the between phases bit.

Between phases, as I mentioned before, rock golem will roll away and you must pursue them. This takes place on a linear track that circles between 2 arenas that you fight the rock golem in. First room is large and open, and has a few stalagmites sticking around the edges. The second room is a bit smaller and offers no natural protection. Third fight takes place in the first room again.
The track itself is long and takes a few twists and turns, and is quite wide to allow for the rock golem to roll around no problem. Players must pursue the rock golem on these winding tracks and not be caught by the hot steam boiling up after the rock golem's weight pushed them down. The rock golem also drops rubble in the player's path, slowing them down or potentially hurting them.

At each "arena" there is a party button that must be pushed to continue to the next wave. The rock golem will remain in an invincible ball form until players step on the button and magma pours up from under the golem, provoking it into fighting us again.

After defeating the golem and its two vog cubs, the original squad meets up with us to congratulate us. However, the volcano starts to shudder - the walls are collapsing! Enough rocks fall in to create a new path, and the final chase sequence starts - only this time you're escaping! The original NPC knights will have made enough of these shields to keep up with you, and will follow any surviving players with about 1 tiles of distance between knights. If any players have gone down and not revived since the Vog Cub battle, these NPC knights are playable, as there are 3 of them in total, meaning one player and 3 NPC knights could escape. If all players are alive, then these knights will do the whole follow-players thing, and if a player is solo, then all three knights will follow the player, doing the exact same moves as them. Upon escaping players find themselves in a danger mission-esk rewards floor, and after this they complete the mission.

TL;DR - big tortodrone-inspired boss monster with 3 phases in a volcano, and players are forced to use an unlimited striker boost for all of the battle. Between phases after after the final one, players engage in a high speed chase sequence while avoiding rocks and rubble.