4/5 stars. The rules are good.
Background.
Devoted to their legion and sworn to protect their leaders even unto taking the death blow for their praetors. The companions are the longest serving veterans of the Deathwing and come with a variety of upgrades, including terminator armour [via Legacies].
Strengths.
WS=5, 2 wounds, and 3 attacks as base (with T=5 for the terminators) makes this a close combat unit.
All members of the unit have Eternal Warrior which improves their survival rates even more - and is appropriately fluffy here. The also have the Companion special rule. This means that the hero that they are accompanying cannot be the target of precision wounds in any shooting attack. This is reasonable, fluffy, and will keep their escort alive a long time against certain legions and opponents who like decapitation strikes.
For the power armour options, the Cytheron Pattern Aegis gives an invulnerable save and the Shieldwall advanced reaction. Certainly worth considering. The calibanite warblade is good, and the option for a terranic greatsword tempting. Powerfists are similarly workable. Melta bombs might not be so recommended, as other units can do it better. Plasma pistols are likely not worth the points sink.
For the terminators, you will want the grenade harness. Again the greatswords are good as are the power fists. I do recommend some thunder hammers here though. Combi-weapons maybe not since this is a close combat unit, but lightning claws may be attractive.
Weaknesses.
Take a land raider for transport and you are probably sorted. I'd also recommend maximum size squads where possible, but this may not always be possible depending on your movement solution.
Builds.
Deathwing Companion Detachment, 10 members, 3 Greatswords, 2 Powerfists, 3 Cytherons (380 points).
The power armour variant has a mix of threats and the Shieldwall possibility.
Deathwing Cataphractii Terminators, 5 members, 5 Thunder Hammers, grenade harness (295 points).
Smaller squad for escort duty and killing stuff.
Deathwing Tartaros Terminators, 5 members, 3 Greatswords, 2 Lightning Claw Pairs, grenade harness (285 points).
Take more members to taste.





