Caestus Assault Ram
The Caestus Assault Ram is one of the Imperium's solution to one of the conundrums of space battle when needing to capture an enemy voidship/its crew/its contents: can't shoot it since there's a big risk of blowing it up for good, docking with it is too slow and hazardous and teleporting squads inside is hard and imprecise. So what do they do in such cases? Send manned vehicles to ram the target and deliver a bunch of warriors straight into the voidship's bowels without damaging it too much, of course! (Whether this is stupidly hilarious or hilariously stupid, we'll let you judge. Just know that the Boarding Torpedo has been part of 40k's fluff since basically forever.)
The Caestus is the Space Marine-only version, a vehicle originally developed for boarding other spacecraft. Its magna-melta fires a shot to weaken the enemy's hull, and then the armored prow smashes its way in. Meanwhile, the occupants are held in an inertial dampening restraint system that protects them from the shock of impact; each restraint harness is so big, it can accommodate a Terminator or a regular Space Marine, so the Caestus can deliver a full squad of any kind of Marine imaginable, limited by the number of restraint harnesses aboard (thus ignoring the Bulky special rule). Once the Assault Ram has penetrated the enemy hull, the assault charges disrupt any defenders with a blast of shrapnel, the clamps release, the hatches drop, and the occupants charge into the fray.
Eventually, the Space Marines realized that the traits that made the Caestus so good at assaulting spacecraft made it very well-suited for ground attack as well. The magna-melta doesn't care whether it's melting a hull, a bunker, or a tank, and the restraint system and durable internal structure and hull meant that the Caestus could make steep orbital drops into zones too dangerous for a Thunderhawk or Drop Pod, especially once it had been upgraded with anti-gravity panels like the Land Speeder. Its frontal reinforcement, meant to ram its way into the often metres-thick outer hull of a capital voidship, also made it rather good at ramming pretty much everything else. So now some Space Marine chapters make ground assaults using Metal Boxes falling from the sky like Steel Rain.
The Caestus Assault Ram's model was created by Forge World for their Imperial Armour volumes covering the Badab War. Its rules are also now covered in Forge World's Horus Heresy series, confirming it was in use by many Legions during the 30k era, particularly the Imperial Fists Legion that often treats void warfare as a three-dimensional siege in space, with heavy preference for boarding actions by their Terminators and Breacher Squads delivered via Caestus.
Incidentally shares its name with a sort of Roman fighting glove weighted with iron or lead.
While the quality of the game itself is a matter of opinion, the intro movie for the console game Kill Team showcases the Caestus Assault Ram quite nicely: https://www.youtube.com/watch?v=D2v3KQcjX5M
The Fan series "Astartes" has an epic clip of it's use as well: https://www.youtube.com/watch?v=mfGPMJ8A0QY&t=0s