One of the best things about the future sight expansion set of magic the gathering is that it introduced some very interesting card mechanics that may or may not appear at some point in the future. Whether they do or not though the cards that use them allow for some interesting deck designs and the chance to surprise your opponent.
Tombstalker is a rare demon creature from the future sight expansion set of magic the gathering. It is a 5/5 flying creature that costs two black and six colorless to put into play. Considerably too much for the creature, even a large one except that it comes with a second ability, delve.
Delve is a simple and powerful ability which allows you to remove cards from your graveyard to reduce the cost of the spell. This means that for every card in your graveyard that you are willing to remove from the game this card will cost 1 colorless mana less, leading to the possibility of a 5/5 flying demon for two black mana.
The natural desire is to put this creature out absurdly early, even on the first turn, and with a dark ritual it is possible, but the question becomes whether that is really a good idea. Sure a 5/5 on the first turn is going to be a problem for your opponent but any removal will give them a large card advantage and even if you have done ten damage before they are able to remove it they will likely be able to overcome that.
Instead a better choice is to put this out on the third or forth turn after using a few common black spells such as creature removal or discard. This still gives you a considerable advantage on the battlefield but without giving up nearly as much.
In limited the ability to simply put spells into the graveyard is a little lower since creature removal is more important and other instants are not as common as some players might like, but the value of a good creature is always high in limited and this is a good creature no matter when it comes into play so keep an eye out for it and if you know that you're going to go into black anyway pick it up early.
This is a mechanic that would be fun to see in a future set but may be too limited to really be a major mechanic which makes it perfect for future sight where you can use it on those cards it works perfectly with and not have to try to stretch it to others.