What is it. Well we have no picture. I did find two conversions, and I've posted them here. It is an insect type creation, obviously looking more like a parasite than the scorpion like Talos. WS3 BS3 S5 T7 I4 A2 Ld10 Sv3+. Just from it's stat line you can see it's differences. It is fearless with power from pain and has move through cover. It's also a monstrous creature.
What sets it apart are it's weapons. It comes with a close combat weapon and a Spirit Syphon. A Spirit Sypon is a Template S4 AP3 Assault 1 that if it kills one more more models you can choose one of your units within 12" of the Cronos, which can be the Cronos, that unit gains a pain token.
You can also upgrade the Cronos.Options include the taking a Spirit Vortex, which generates pain tokens like the Spirit Syphon except that it is a Range 18" S3 AP3 Assault 1 Large Blast. The other option is the Spirit Probe which allows you generate pain tokens in close combat, again like the spirit syphon.
So upgraded properly, with both Spirit Vortex and Spirit Probe, the Cronos Parasite Engines able to generate 4 pain tokens in a round. It is a monstrous creature so it can fire both weapons during the shooting phase, a death from each one generates a single pain token. Then in assault if a single casualty occurs it generates another pain token. Then as normal, if you wipe the unit, you generate a pain token for the cronos. Each weapon apparently rapidly ages its victims leaving nothing but a dried out husk of a corpse.
Both upgrades are what should be fielded with the cronos to maximize it's effectiveness. I currently have two of them in my list that you can read (posted a few days back). Two Cronos Parasite Engines backing up a single Talos Pain Engine and a unit of Incubi. While I think the Cronos can be fielded in the backfield a little better than the talos, I suggest using a webway portal to really make the best use of the creature. From midfield the Cronos should really be a threat to your opponent and be able to back up your forward assault units. Watch out for melta teams or plasmas.
The Parasite engine really seems to meld nicely with a talos. The Talos being that close combat death, while the Cronos deals its death in the shooting phase. With a toughness of 7 and a single token that you should be able to get rather quickly, it should have good survivability on the tabletop. While it doesn't have the mass number of possible attacks or the greater strength of the Talos, S5 with furious charge will make it no slouch.
I see them being a strong unit in a list designed with them in mind. Don't simply just place them in your list without really figuring out how to make them effective. First round missle fire will kill them dead. Webway Portals lists seem the best way to get them on the field where they can unleash on the enemy. If you are facing a fast moving assault army like blood angels or space wolves, I can see deploying the Cronos. However portals still seem like the best suprise they can offer. Punish severly those that would approach your portals and attempt to block them. That will be the way I use the Cronos Parasite Engine.