Blizzard gave us the answer we desperately have been clamoring for just moments ago with the following tweet and subsequent hero reveal:

Many people guessed a Kobold hero, Deckard Cain, and even a Starcraft Firebat, but it turns out that the three candles, suspensefully snuffed out one-by-one on the Heroes launcher, were a grim foreshadowing of Malthael, the Aspect of Death from the Diablo 3 expansion, Reaper of Souls.

Malthael is an Assassin hero who, from the looks of it, will deliver an incredible amount of damage. His abilities can be found on the official battle.net Heroes page, or below for convenience.

TRAIT 

REAPER'S MARK
Basic Attacks afflict non-Structure targets with Reaper's Mark for 4 seconds. Marked enemies are revealed and take damage equal to 2.5% of their maximum Health every 1 second.

BASIC ABILITIES

SOUL RIP (Q)
Extract the souls of nearby enemies afflicted by Reaper's Mark, dealing 100 damage and healing Malthael for 44 per target hit. Heroic targets heal Malthael for an additional 3% of the Hero's maximum Health.

WRAITH STRIKE (W)
Instantly teleport through an enemy afflicted by Reaper's Mark, dealing 59 damage and refreshing Reaper's Mark.

DEATH SHROUD (E)
After 0.25 seconds, unleash a wave of dark mist that applies Reaper's Mark to enemies it hits.

HEROICS

TORMENTED SOULS (R1)
Gain 20 Armor and unleash a torrent of souls, continually applying Reaper's Mark to nearby enemies for 4 seconds.

LAST RITES (R2)
Apply a death sentence to an enemy Hero that, after 2 seconds, deals damage equal to 50% of their missing Health. Repeatable Quest: Enemies killed while under the effect of Last Rites permanently reduce its cooldown by 5 seconds, to a minimum of 15 seconds.

From the looks of it, Malthael looks capable of dealing an incredible amount of damage. Most noticeably from his trait that, every second for 4 seconds, deals damage equal to 2.5% of a Heroes maximum Health. This promises to make Malthael a capable solo-laner while also excelling against high health warriors, similar to Tychus and his percentage based trait.

Malthael also has the first heroic that comes attached with a quest. This will likely give Malthael capable late-game snowball potential, as multiple quest completions allow give me more sustained damage and hero kill potential.

Talents, release date, and PTR schedule are still unknown, and will be reported on when available.