This is RainbowMustache22 's OC, Darkpine

Name: Darkpine
-pine is not a very common suffix, so I like that! Dark- is pretty overused, but in this case, it's alright. This name matches his appearance, so great job! Although, one thing: dark- is a pretty evil-like prefix, so it's a bit stereotypical.

Looks: Dark brown with a white tail tip. Has dark green eyes.
It's pretty plain, but I like him. Green eyes are pretty common, so maybe you could change it to yellow-green if you want, but green eyes is still acceptable on him. And one thing: like I said about his name, it's pretty bad-guyish, so there are a few stereotypes here.

Personality: Controlling, but is also fierce and brave. He is cold and doesn't say much, but is loyal to his Clan through and through.
If he's loyal, why did he go to the Dark Forest? Otherwise, this is a pretty good personality.

Gender: Tom
*dances happily*

Backstory: When Darkpine was an apprentice, his mentor was the Clan deputy. Darkpine was very scrawny weak as an apprentice. His mentor didn't believe he would ever be a warrior, and made sure Darkpine knew this. When the time came for Darkpaw/pine's assessment, his mentor purposely told the Clan leader that he had failed his assessment and wasn't ready to become a warrior. Shortly after, his mentor died in a battle against ThunderClan, and Darkpaw/pine was given a new one. Shortly after that, Darkpaw/pine was made a warrior. Darkpine was no longer the scrawny and weak apprentice he was before,
for Darkpine had started training in the Dark Forest. Darkpine knew what he was doing was wrong, but he didn't care. Darkpine fell in love with another cat, Mapleleaf. Mapleleaf was pregnant with Darkpine's kits soon after. Mapleleaf did not know of Darkpine's plans to kill the Clan deputy, and then the leader, so he could take control of the Clan. Mapleleaf had kits, but Darkpine didn't want them, didn't love them like a father should. Darkpine killed the leader, and took over the Clan. Mapleleaf took the kits to WindClan, where they would be safe from Darkpine's treachery. Darkpine was furious. He murdered Mapleleaf. Eventually, with the help of the other three Clans, Darkpine was chased out of the forest. The other three Clans had worried he would take over their Clans too.
You're typical bad guy. I like this backstory; it's not too sappy, not too sad, not too perfect, and not too gory. It's petty sad, though, the way he murdered his love and left the kits motherless.

Extra: none
That's fine.

Cause of Death: Badger killed him. He came back to the forest, and tried once again to take over his former Clan. Mapleleaf's kits, now Windclan warriors, chased him into a badger's den.
Wow. Revenge, huh? Badger deaths are semi-common, but you made this one unique by making his kits chase him in. Nice job! :3

Mate: Mapleleaf (formerly)
Sad.

Kits: Shadowclaw, Blossomtail, and Frostberry.
I like the last two names, but shadow- is not a prefix ever. I don't like it when OC's prefixes are either shadow-, thunder-, river-, or wind- because that's what separates the Clans. If a cat with these prefixes became leader, they would be Shadowstar, Thunderstar, Riverstar, or Windstar, and those are the names of the founders if the four Clans. So, maybe change that.

Clan: ShadowClan
Once again, the ShadowClan stereotype.

Rank: Warrior (formerly), Leader (formerly), rogue
Yes.

Afterlife: Dark Forest
Obviously.

Overall review: 9.9/10
He is awesome. It's just those stereotypes in there and the shadow- prefix that bugs me. Maybe change it up a bit, and you've got yourself a perfect OC!