Antaro Chronus is a tank commander of the Ultramarines Chapter of Space Marines in Warhammer 40,000. He is a Brother-Sergeant of the Ultramarines' Armoury and holds the title Spear of Macragge. As is true of almost all of the Ultramarines' special characters (besides Ortan Cassius and, to a lesser extent, Torias Telion), Chronus is a goddam Mary Sue that's absolutely perfect at everything ever (well, everything related to piloting a tank, at least).
Chronus seems to rub non-Ultramarine players off the wrong way. This is partly because Space Marines is not, traditionally, an army heavy on tanks, but it's also because he's overshadowed by Knight Commander Pask, an Imperial Guard tank commander that's far more awesome. He also wasn't introduced until Matt Ward's Codex:
Ultramarines Space Marines was released, so he's (relatively) new.
Those that do know about Chronus are usually pretty mad about him, for a few reasons. First off, he's stated as excelling at every vehicle in the Ultramarines' armoury, despite the fact they all have very different battlefield roles. Apparently he's just as good at aiming a Whirlwind's barrage as he is at piloting a Predator. He also kept the Predator Rage of Antonius running during the Damnos Incident, despite the severe damage it had sustained. This baffled the Chapter's Techmarines, until they realized that Chronus had extreme Plot Armour and should probably be promoted. Finally, the Ultramarines got ANOTHER special character, even though the Iron Hands still don't have any. This is especially rage-inducing, as the Ultramarines aren't especially known for their vehicle formations or technical skill; Chronus would fit much, MUCH better as an Iron Hands or Aurora Chapter character (it should be noted, however, that the Aurora Chapter is a successor of the Ultramarines. So there are some treadhead genes in Gene-seed of our Spiritual Liege. Make of that what you will) . But no, the Ultramarines needed another Mary Sue special character.
He also showed up in Gathering Storm 3 where he took part in a tank battle on Maccrage after the awakening of Roboute Guilliman.
When he was first introduced, he didn't do much crunch-wise. Whichever tank he piloted got to use his Ballistic Skill of 5 and... well, that was about it. For 70 points. Oh, and if the tank blew up, he had a chance of hopping out and becoming an Independent Character, or, as we like to call it, a "free kill point." Nowadays, he's cheaper, and as shown below, he's actually worth something now.
On the Tabletop
Sergeant Chronus does 3 things. First, he provides a budget HQ slot if you want one, as he's only 35 points. Second, he buffs a tank quite considerably by fixing its ballistic skill at 2+, making sure the tank will always shoot at maximum efficiency, regardless of damages taken. Third, he heals one HP every turn, because you know, he can fix the tank and be a crack shot at the same time.
Since 8th does not gives buckets of re-rolls anymore, with twin-linked being effectively replaced by twice the shots, put Chronus in a Land Raider for 4 pinpoint accuracy Lascannon shots a turn, not taking into account other weapons. With "Power of the machine spirit", nothing short of rare psychic powers will deny you that 2+ BS.
If his tank is destroyed, however, he is pretty useless as he only has a Bolt Pistol and Servo-arm (basically a Thunder Hammer that can only attack once per turn)to fight, and no ability to boot, so make him retreat to the nearest objective and help it hold it.