"Let Them Weaken It" is a thing that happened where ED let Cursed Brotherhood weaken a mob before ED went in and ganked it.
Edit by Odeous Ogerr: Incorrect. "Let them weaken it" were the words shouted zone wide by the then leader of the Clan of Shadows, Mackaveli, Barbarian Warrior, who was unhappy at the time with the constant loss of named spawns to the larger and more well staffed gulid Enlightened Dark.
I did not wan't to full change your initial entry. If you find my edit to be correct. I leave that to you sir.
SECOND WAVE GO!
Regan edit: It was faydedar, the Tmorous Deep Dragon, before it was made spawnable on that ringed island as part of the Druid epic.
Both CoS and the oposing guild (rather than ED, I thought it was DoA...) claimed to have 'first agro' as Mack himself had been kiting the dragon around the ocean along with a bard from DoA. Both guilds were formed up on oposite ends of a small island with both pullers attempting to lure the dragon to their guild's camp. While competing for agro (whoever is closer is who the dragon will follow) Mack got dispelled and died. It was while Mack was being rezed/rebuffed etc that DoA engaged. This prompted the infamous 'Let them weaken it' moment. CoS believed that by virtue of having agro on the dragon already, they had every right to engage it, and did not wait for DoA to wipe. The CoS wizards were instructed to 'nuke at half life' which allowed them to complete the KS.
Brael edit:
Ok, I was there and remember more or less what happened from the CoS side. The opposing guild engaged Faydedar in Timorous Deep, it was Macks belief that that they would wipe or lose enough of their dps in the fight that a kill steal would be possible. This belief was rooted in the idea that both the other guild and CoS had been forced to kite for quite some time because neither guild had enough people to kill it. Once the other guild went for the kill the idea was to keep CoS back but engage after the other guild had expended most of their resources on the kill. What actually happened however was that when the other guild engaged Faydedar CoS initially ran in to compete for a legit kill steal LET THEM WEAKEN IT (which was spammed many times) ended up being a call to stop engaging and wait a minute before attacking.
The velious turtle people are mentioning in the comments was a series of seperate events with Eduin who would continually attempt (and sometimes succeed) at kill stealing by training the indegenous gnolls onto the turtle and then pocket the loot.