Aside from Sombra not seeming anything like a planeswalker to me, this could totally work with a few modifications.  
First, he’s stupidly overpowered for a 3cmc walker. This is definitely more of a 5cmc card.  
His +1 is WAY too strong for a +1. Change those 3s to 1s and it’s golden. Not too strong, but just good enough to always be handy.  
His -2 is good, but try changing it to “Target creature gets -X/-X until the end of the turn where X is your devotion to black”. Would probably just fit better with actual card effects.  
Finally, his -7 is absolutely ridiculous for its cost coupled with the cost to cast Sombra himself. Replace “sacrifice” with “destroy” and it becomes much more reasonable. If you want to keep a sacrifice effect, the cost either has to be increased drastically or the sac conditions need to be a bit more under your opponent’s control. Take for example Liliana, where you divide their permanents, but your opponent picks which pile to sacrifice, or Bolas, where they sac a set number as opposed to everything on their field that isn’t a land.

Original is here: >>152230

The +1 is way too strong. I think it’d be fine at 1 life. 2 might be pushing it, but doable if you’re planning on raising the CMC.
 
-2 is good. Don’t mess with it.
 
-7 should maybe be a -8, but otherwise, it’s a great ability.

Card looks beautiful and has excellent formatting, however it does have balancing issues. My thoughts?
 
Change cost to to 2BB  
+1 ability to 2 life each  
-7 ability to destroy all other creatures

1: King Sombra doesn’t strike me as a Planeswalker.  
B: He’s a bit overpowered for only three mana. Nicol Bolas costs seven, and his ultimate technique makes them lose seven life, seven permanents, and their hand (unless the have more than seven in hand). Or, instead of increasing the mana cost, you could reduce the starting loyalty (Example: Tibalt, the Fiend-Blooded).  
III: I’d switch the first two abilities, since killing a creature with -1/-1 is useful, but not necessarily a win condition, while making someone lose 3 life could be all you need.

While the mana cost of the planeswalker does matter, generally the +1 should be something that you’d print on a 1 mana instant, and the minus abilities should cost loyalty about equal to the CMC of a sorcery that does the same thing (Or an enchantment in the case of an emblem) For a black, blue or red planeswalker, the starting loyalty should generally be one less than the planeswalker’s CMC. A green or white one should be equal.

Up the cost to 2BB or make his starting counter amount 3.
 
That first ability… Routinely biting 3 life out of someone and adding 3 to your life total starting on the third turn like that is nuts. Given that Sombra’s presence is sort of felt en-masse, I’d make it “+1: Each opponent loses 1 life. You gain life equal to life lost in this way.” It’s a spread effect, but the impact isn’t as detrimental at the outset.
 
Second ability…actually that’s pretty good. It’s not a guaranteed creature kill, but it’s suitably frustrating.
 
That third ability’s a bit overpowered. It’s either an early killer, or if you’ve got the right stuff around him, he’s a mid-game nuke. Furthermore, it’s a sacrifice effect, which is incredibly hard to avoid.
 
Admittedly, I like the idea of his final ability being some sort of all-consuming plague. Maybe make it “-7: Destroy all creatures you don’t control. They can’t be regenerated.” It’s a Plague Wind effect, and that’s really what Sombra is.

Plus one is WAY too strong. That’s a Lightning Helix with lifeloss rather than damage.

Love the templating. The +1 is too strong. I’d make it a little more under your opponent’s control. Maybe “Until your next turn, whenever a creature deals combat damage to Sombra, that creature’s controller loses that much life and you gain that much life.” The second ability is good if the set he’s in cares about color. I’d either make him cost 2BB, make his starting loyalty 3, or make his second ability -3. His ult is a little simple. I’d make it “-8: Destroy all creatures, then draw a card for each creature destroyed this way.”

@Sixtyfour: He’s just an average Planeswalker. Since planeswalkers are considered players does that mean King Sombra can be on my side?!

dear wizards……MAKE THIS REAL

Huh, the cards looked nothing like this last time I played.
 
Why is there only one number in the lower left?