Pull of the Abyss:
At the beginning of your upkeep, you may sacrifice Pull of the Abyss. If you don't, you lose 2 life.
Whenever a creature dies, if it's the first creature to die this turn, put an Abyssal counter on Pull of the Abyss.
Creatures get -0/-X, where X is the number of Abyssal counters that were on Pull of the Abyss at the start of the current turn.
If the wording seems odd at some points, it's because it took a lot of fine-tuning to get the card to work exactly the way I wanted it.
The original concept was simply "whenever a creature dies, put a counter -> creatures get -X", but that approach would snowball far too quickly when there are a lot of low-toughness creatures on the field.
I decided the best way to smooth out the power curve was to have it so that counters could only be added once a turn. At first, I simply went with "at the beginning of each end step, if a creature died this turn", but I wanted to avoid corner cases where the ability simply wouldn't work -- creatures dying after the beginning of the end step when it was too late to trigger, or the end step being skipped entirely.
I tried changing it to "at the beginning of each upkeep, if a creature died last turn" -- after all, it worked for werewolves. But that just got even more awkward when considering the possibility of multiple upkeeps.
So I finally settled on having the counter added immediately when a creature dies, and making the static effect only change values once per turn. This can result in the odd game state of having a 0-toughness creature alive for the duration of the untap step, but it doesn't result in any rules shenanigans, since it will just die the next time SBAs are checked. At which point, it will add another counter, which is why I only wanted the value of X to change once per turn; if it updates immediately, then you lose that chance at a free counter -- it also gives your opponent the chance to drop something that will raise the creature's toughness, giving more options to play around and interact with the card.
I don't consider myself to be a Vorthos, but I really like the flavor here. Every time you kill a creature, the pull of death grows even stronger, slowly drawing in everything. Even the upkeep cost - apart from allowing you to ditch the card if it becomes too troublesome - represents the player himself being slowly pulled into the grave - and is specifically worded the way it is to give players the option of killing themselves with it, if they so desire. After all, summoning a portal to the void to suck in your enemies at the risk of being sucked in yourself feels, to me, like a very thing to do.
Originally, I thought would be an appropriate mana cost, but that was before I added the once-per-turn clause. WIth that in place, I felt a four mana cost made it too slow to gain any benefit from in competitive play. I considered dropping it down to to bring it in line with some of the Zendikar quests. With the upkeep cost offsetting the immediate-and-increasing benefit compared to the delated-but-static benefit of quests, that seemed like a reasonable cost for a 1V1 environment, however, the more players there are, the faster this thing can grow. I felt the extra was necessary to keep it reasonable in multiplayer environments, while still giving the option of powering it out with a Dark Ritual .
Given that wall of text, I probably don't need to state that I've put more thought into this card than I have most others I've come up with.
But I stated it anyways.