For some time, Skullbriar has been about a $6-7 card who sees a ton of play in the Commander format. However, in late March 2015, an announcement was made that will change the Commander format forever: Commanders originally could only return to the Command Zone whenever they were destroyed or exiled. They could still be sent back to the library or to their owners' hands. Due to a new rule change, players can choose to have their Commander return to the Command Zone instead no matter how the Commander is removed from the field.
"Tucking" Commanders has long been a part of the Commander format. Many players aren't happy about this change, since some Commanders are best dealt with by returning them to the deck. Now this isn't going to be possible. Returning to the hand isn't too big a deal for most Commanders, and some players prefer this, since you can recast the Commander for its original cost from the hand, However, in the case of Skullbriar, the only way to reset his +1/+1 counters was to send him to the library or the hand. Now, the Skullbriar player can elect to simply return him to the Command Zone. Sure, you'll pay an additional 2 colorless mana for each time he's been cast, but you get to keep his counters. Plus, he has Haste to begin with, so you don't even lose a turn of attacking with him.
Now that Skullbriar can go back to the Command Zone any time he would leave the field, his greatest weakness no longer exists. It makes him a lot, lot scarier, and players who felt his weakness was too great a drawback before suddenly want to play him. Since Skullbriar was only available in a pre-constructed product, and an older one at this point, his price has spiked ridiculously. The beauty of Skullbriar is that really all you need for him is ways to protect him and get him through unblocked (or give him trample), while supplementing him with Zombie tribal support (as he is a Zombie Elemental). You have all of the green mana ramp you could dream of, and the black removal to make sure Skullbriar and his Zombie hordes have plenty of room to deal massive damage.. Cards like Doubling Season are also quite beneficial to stacking up his counters more quickly.
While many Commanders will benefit from the Command Zone rule changes, none benefits nearly as much as does Skullbriar. If you still have a copy of him hanging around, you may want to give him a shot as your next Commander. If you've already been playing Skullbriar, congratulations: things have just gotten a lot more fun for you! Skullbriar, the Walking Grave is now one of the top Commanders in the format.