The Red Scorpions are Forgeworld’s flagship Chapter, and have been included in a number of Imperial Armour books. In their current iteration, the Red Scorpions are a proud Chapter that is obsessed with maintaining the purity of their gene-seed. On the tabletop, and Sergeant or Veteran Sergeant leading a Tactical squad gets a narthecium for free, granting his squad Feel No Pain (5+). If you plan on taking loads of Tactical Marines, this trait is amazing. It’s more limited in scope than the Iron Hands trait but twice as effective.
In addition, the proud Red Scorpions refuse to cower in the dirt; the re-roll failed Pinning tests but cannot voluntarily Go to Ground or take camo cloaks. These rules are exceptionally fluffy and not too restrictive, unless you take Scouts.
Red Scorpion armies will be most effective swamping the enemy with masses of Tactical Marines, which are both more durable and more reliable than standard. Beyond that, they can take a well-rounded army without any loss, though I would avoid Scouts as they’ll be hurt by the inability to increase cover saves.
Lord High Commander Carab Culln
First, Carab Culln is one of the select group of special characters with Eternal Warrior, which is enough to justify his use. His Master of Battle rule allows all squads in his detachment to use his LD10 for pinning and morale tests, providing a very useful army-wide buff. Finally, he has the awesomely named Warlord Trait Proud to Live, Proud to Die, Hard to Kill. This isn’t really that amazing, as it’s a close-combat buff that adds +1 to the result of any assault within 12” of Culln, and +2 if he’s engaged in a challenge in that assault. However, it does prevent him from using Look Out, Sir. Not a huge concern given his durability, but you’ll have to be more careful about his placement than most characters.
Carab Culln is a well-rounded character that fights well in close combat while providing some buffs to his army. He’s not the most amazing choice, but he’s still quite useful in the same sort of roles as Lysander or Calgar.
Magister Sevrin Loth
Sevrin Loth is the Chief Librarian of the Red Scorpions, and provides an alternative to Tigurius for players looking for a strong psyker. He has a slightly stronger statline than a normal Librarian, and is equipped with the standard gear. He wears the Armour of Selket, artificer armor that grants a 2+ invulnerable save each turn if you expend a single Warp Charge to power it. You’ll have to be careful using this so Loth doesn’t get Instant Deathed, but this is an incredible defensive ability.
Most importantly, Sevrin Loth is a Mastery Level 3 psyker with access to Biomany, Telepathy, and Telekinesis. Even better, he picks any three powers he wants rather than rolling randomly, but is limited to a single discipline. If you need a specific power for your army to function (and it’s not from Divination), Loth is the psyker for you. He also has Fearless and Counter-attack just for the heck of it. His Warlord Trait grants a Victory point for each psyker character he slays over the course of a battle. Usually, these traits aren’t great because they are easy for your opponent to avoid and put your Warlord in danger, but this is a bit better. With his force axe and 2+ invulnerable, Sevrin can handle himself in a challenge, and there are often quite a few psykers on the table for him to hunt.
Magister Sevrin Loth is a solid choice for a strong psyker, and is a surprisingly dangerous close-combat character. He is most useful if you’ve built your army around getting a single psychic power, which he can guarantee.