Warpstone Flux Rating: ⭐️⭐️⭐️⭐️
4/5 stars. The rules are great - he is a tremendous beat stick!
All of the legions have recognized "names" that their brothers and cousins will instantly know. For the Dark Angels, Corswain's name is one that echos still some ten millennia later such was his skill.
Honestly, if you want something dead short of a Primarch, then Corswain is your man. And he might even have a shot at some primarchs as well. I like to think of him as a beat stick legion champion. He has the kit to boot: The Blade has both duellist's edge and murderous strike along with being master crafted. His armour is insane for keeping him in the game by providing 3+ invulnerable in close combat and 4+ otherwise (or a regular 2+ save). Mix in that he has WS=7 and precision striking at 4+ and he will get the job done.
The first weakness is that if he is your warlord, you get locked into Marshal of the Crown (Death wing). It is not bad, but there are much better alternatives.
He's loyalist only, so a non-starter for traitor lists.
He kills stuff in close combat while staying alive very well. That's literally all he does. He does it to a maximum standard, to be clear. But he doesn't provide army buffs or anything else. A true beat stick in the traditional sense of those words.