Reckoning is a paladintalent in the Protection tree; you will need to spend 25 points in Protection to max it. This talent gives the paladin a chance to gain an additional attack on their next auto-attack, stacking up to 4 times.
A "Reckoning Bomb" is an attack that consumes multiple charges of Reckoning at once, dealing a burst of damage via consecutive attacks. This is done by turning off auto-attack and allowing yourself to be critically hit until multiple stacks are accumulated, then turning it back on to use them all.
Reckoning could at one point store an infinite number of charges, which meant Reckoning Bombs had the potential to deal massive amounts of damage. In May 2005, a paladin named Karmerr soloed Lord Kazzak in the Blasted Lands by unleashing a 1,816-charge Reckoning on him, killing him in a matter of seconds.
Shortly after this, Reckoning was hotfixed to store a maximum of only 4 charges.
Trivia
The Argent Cannons in Icecrown have an ability called Reckoning Bomb that inflicts "ridiculous amounts of damage."
Reckoning appears as an epic Paladin spell in the 2021 Core set for Hearthstone. The artwork depicts the Reckoning Bomb, and its flavor text reads: "He would henceforth be known as 'Karmerr, the Doom of Kazzak.'"
Patch 4.0.1 (2010-10-12): No longer deals damage if the target is not currently targeting you. Moved to tier 4 (was tier 5) and slightly redesigned: You have a 10/20% chance after blocking an attack for your next 4 weapon swings within 8 sec to generate an additional attack.
Old: Gives you a 2/4/6/8/10% chance after being hit by any damaging attack that the next 4 weapon swings within 8 sec will generate an additional attack.
Patch 2.2.0 (2007-09-25): Can now trigger while the paladin is sitting.
Patch 2.0.1 (2006-12-05): Redesigned: Gives you a 2/4/6/8/10% chance after being hit by any damaging attack that the next 4 weapon swings within 8 sec will generate an additional attack.
Old: Gives you a 20/40/60/80/100% chance to gain an extra attack after being the victim of a critical strike.
Hotfix (2005-05-12):We have hotfixed the Paladin talent Reckoning to stack a maximum of 4 times.
Patch 1.3.0 (2005-03-07): Fixed a bug where abilities that give extra attacks, like the paladin Reckoning talent, could cause the following swing to take longer than it should.
Patch 5.0.4 (2012-08-28): Removed. 
Patch 4.0.1 (2010-10-12): No longer deals damage if the target is not currently targeting you. Moved to tier 4 (was tier 5) and slightly redesigned: You have a 10/20% chance after blocking an attack for your next 4 weapon swings within 8 sec to generate an additional attack.
Patch 2.2.0 (2007-09-25): Can now trigger while the paladin is sitting. 
Hotfix (2005-05-12): We have hotfixed the Paladin talent Reckoning to stack a maximum of 4 times. 
