And I looked, and behold a pale horse: and his name was Death, and Hell followed with him.
Death is one of the four Horsemen of the Apocalypse. He is a boss which can be encountered at the end of The Womb and Utero 1 once the player has beaten the game for the first time (killed Mom). He looks like an eyeless, gray humanoid-like being with a skinny, pale grey body and a hollow skull, riding a skeletal hobby horse. His blood is also completely black. He will surround himself with the Grim Reaper's trademark weapon, the scythe.
Like the other three harbingers - Pestilence, Famine and War - Death leaves a Cube Of Meat upon defeat which cannot be rerolled with The Dice.
At first, Death rides on his horse, attacking by either summoning a quartet of scythes that travel across the room or summoning a pair of scythes that chase the player. With careful aim and enough firepower, the scythes can be shot down, which may be easier than dodging.
During the fight, Death is also capable of using the item The Hourglass against the player, reducing movement speed and slowing down tears for a few seconds.
When the boss reaches 50% health, he dismounts from his horse, which constantly sweeps horizontally across the room in an identical pattern to War's charging attack. The horse has a relatively high amount of hitpoints and damaging it will not reduce Death's health bar, but must still be destroyed along with any summoned minions to finish the battle.
The unmounted Death will summon pairs of Knights to attack the player, with a small chance of spawning them as Selfless Knights. They will be killed if his horse happens to run into them. The horse may get hit upwards or downwards if it hits a knight at a certain angle.
Black: Is slightly larger and has more health than normal Death. He always spawns 4 scythes and instead of using the Hourglass, he spawns two Kamikaze Leeches. While separated from his horse, he periodically spawns two Red Maws instead of Knights.
Death's summoning of whirling scythes (and possibly even his use of the Hourglass item) is most likely a reference to the Castlevania series of video games, where the Grim Reaper (also under the name of Death) features as a boss and uses an identical scythe-shot attack in every Castlevania game he appears in.
The Black version of Death may be a reference to the Black Death that plagued Europe during the Dark Ages.
One of the effects of the Black Death was severe blood build-up in some victims, explaining the Red Maws (that are filled with blood) that Death spawns. One thing plague victims were treated with to prevent the blood build-up was leeches, explaining the Kamikaze Leeches Death spawns.
Death's head resembles the 'planet face' of Chapter 6: The End in Super Meat Boy, which also appears as a skull.
Death is the only boss able to use items that Isaac can find.
It seems that Death can sometimes be pushed too far through the wall and then cannot be killed and will continue summoning scythes until the player loses. This glitch can actually happen to all Horsemen. However, Conquest, War and Famine can break out because of their dash attack. Death and Pestilence lack it and will essentially cost you a life if you push them into the wall. However, it's simple enough to avoid.
There is a rare graphical glitch where the summoned scythes will look like Death instead of their normal appearance. (They still act the same as regular scythes).
When Death uses his hourglass attack, if the player has spectral tears, the player will lose spectral tears while the Hourglass is active.
In The Chest, Death's Scythes can knock Isaac through the door.
There is a rare occurrence where, in the chest, Death will not attack Isaac until Isaac attacks him. (It is unknown if this is possible in The Cathedral)
Death will still use the Hourglass though.
If Death is killed very fast (e.g. Mega Bombs), his Stickhorse will continue to charge around map. There is no health bar for it. However it can still be killed.