The converted mana cost for the Tombstalker is eight. You will need two black mana and six colorless mana to bring Tombstalker from your hand onto the battlefield. However, the casting cost for the Tombstalker is quite expensive. Most players would not bother putting Tombstalker into a deck because of the insane casting cost. For a simple 5/5 creature, it seems to be unreasonable to pay eight mana. But, there is more to Tombstalker than meets the eye.
What does Tombstalker do in the first place?
The Tombstalker has Flying. Should you attack with Tombstalker and your opponent has creatures with neither Flying nor Reach, s/he is going to be taking five points of combat damage. In this respect, you could swipe a chunk of your opponent's life points in a matter of turns if s/he cannot depend against the Tombstalker. One could say that a 5/5 creature with just Flying does not warrant such an expensive mana cost. But, Flying is not the only ability of the Tombstalker.
The Tombstalker has this ability called Delve. In this respect, you may Exile any number of cards in the graveyard from the game. You do this as you are casting Tombstalker. For each card that is Exiled from the game, you reduce the casting cost of Tombstalker by one colorless mana. In this respect, you can Exile up to six cards in your graveyard from the game. Afterwards, you simply have to pay two black mana. Instead of paying eight mana, you are only paying two black mana.
Being able to bring out a 5/5 creature with Flying for only two black mana is a great thing. While the initial converted mana cost for the Tombstalker is eight, you can reduce the casting cost to two with the Delve ability. In this respect, you can bring out the Tombstalker and still have plenty of mana left to use. This is considered to be the best part of the Tombstalker. Normally, a 5/5 creature with Flying would cost between five to seven mana. With Delve, you get to cast Tombstalker for only two cards.
Overall, Tombstalker is a good card to have if you are playing any format that is not standard. A 5/5 creature with Flying for only two black mana is a great deal. All you have to do is Exile six cards from your graveyard.