Atrium is a boss that was added on July 15, 2019. Later on, it was added secretly into the game.
Atrium has a white triangle base with three drone spawners on each side and a Constructor cannon on top of the base.
Atrium has incredibly low health, (lower than ANY boss, testbed or otherwise while the only exception is industry), but fast speed. It has very powerful ammunition, blocks have a lot of penetration, damage, and speed but low health. Drones are very fast, have decent health and penetration, and do medium damage.
Spawns by a randomizer.
When countering Atrium, remember, it has very low health, so charging is very effective. However, be careful. As a bullet spammer, you will not be able to penetrate the ammunition of Atrium, and as such it will kill you very fast. If you are a bullet spammer, it is the best option to run away in order to wait for boss to die. As a Pounder branch tank, you should get closer to Atrium and shoot your powerful bullet. Boss will die from a single bullet, so hit it by all costs. As a drone or minion user, charge all of your drones into the Atrium, and it will be dead. As a Trapper branch, you need to retreat, while creating a line of traps so that Atrium will hit all the traps and die.