"Sir, scanner's got something fast and moving this way."
Sergeant Urkel Steele of the Imperial Guard readied his chainsword as his men dug into this fortification built into a hill near the Tau front lines. Several Basilisk cannons had been built into this place, with Chimeras positioned at the bottom. He had just heard that now the forces of Chaos were launching an incursion into this world—the flashes in the sky, clearly originating from lance exchanges in orbit, obviously signified this. With all these heretics, xenos, and demons infesting this world, he wondered if they could conceivably hold. But then, they were the Cadian Imperial Guard, the finest of the Emperor's mortal sons.
But now, he had heard of a single demon that could potentially threaten even his forces. A demon that had killed a Space Marine officer. A demon that destroyed another Guard position. A demon that had crippled a Tau forward force. A demon that had laid waste to an Ork band. From what Warp-spawned hell this abomination originated from he did not know, but he had been paying enough attention to comms chatter to know that it was there.
"Sir! Scanner's got something fast! And moving this way!"
"What? Oh." Steele snapped out of his thoughts as Guardsmen surrounding him sighed and rolled their eyes. "What is it?"
"Profile matches that of a Tau battlesuit. However, there's only one, and its movements are...erratic, to say the least."
"Tau suits always travel in groups and their movements are very linear. Either the pilot has been exposed to the more unhealthy minions of the Ruinous Powers, or..."
Fusion bolts came out of the distance, blasting apart the Chimeras. A faint screeching sound, like a Slaneeshi cultist slowly getting a chainsword jammed up its rear end, grew in pitch.
"What in the Emperor's name is that?" uttered Steele.
"It sounds like you upon looking on an enemy force greater than a few Orks, sir." Said one of the Guardsmen.
"Normally, I would kill you for that, but I'm afraid there's no retort to that." Sighed Steele in admission as the Guardsmen chuckled.
Then, a Tau battlesuit entered their field of vision—and just as the scanner operator had reported, it was moving very strangely indeed. It was zig-zagging across the landscape, dancing like a drunk Ork, and occasionally bursting forward with its jetpack and randomly firing its weapons. Steele could faintly hear a squeaky voice coming from its communications speakers:
"Aaaaaieeee! How do yousa control thisa piece of junk! Help meeeeesa!"
"Clearly, Tau standards have been dropping lately." Chuckled Steele. "Target that suit and open fire!"
The Guard opened fire, but the suit was moving too erratically and quickly for them to get a proper target acquisition. The lasgun shots that did hit failed to penetrate the suit's thick armor. Steele watched in horror as the suit's jetpacks activated and it shot forward, landing right in their midst.
"Disgusting xeno, get out of here!" he shouted as the suit simply stood on the spot and continued dancing, with lasgun fire harmlessly impacting on it. He then gestured to cease fire, and watched as it continued dancing, shaking its backside and flapping its arms. The Guard then burst out in laughing, with even the snipers with them chuckling at the sight of a Tau suit, to use the best possible phrase, 'shakin' that thang'.
Suddenly, the weapons of the suit fired up and shot out, blasting apart a dozen Guardsmen and splattering their innards against the rock. The suit then stepped back, crushing more troops under its feet, before randomly firing its weapons, blasting apart the fortification.
Cursing himself at letting his guard down—in every sense of the term—Steele charged at it with his sword, only to get knocked back by a kick, shattering his ribcage. As he lay on the ground, dying from severe internal bleeding, the suit opened up and spat out a gangly thing that brushed itself off and walked away like nothing had happened.
Steele retained conscious enough to feel furious that his men had been beaten by that stupid-looking thing, and glee that a group of Space Marine Terminators had been reported heading this way.