Trials of the Naaru (aka Trials of your tanks' patience) was a series of quests for TK attunement.

The chain started with a quest titled "The Hand of Guldan" in Shadowmoon Valley and ended with several parallel chains that ultimately led to a simple 5 man boss. On completion, you got a letter from A'dal who then sent you on three parallel quests known as the Trials of the Naaru: Mercy, Strength, Tenacity.

Mercy required you to complete heroic Shattered Halls in 55 minutes from starting the gauntlet. You had to rescue a prisoner at the end.

Strength required you to complete heroic Steamvaults and Heroic Shadow Labs and get drops from the final bosses.

Tenacity required you to save Millhouse Manastorm in Arcatraz.

After doing all three quests, you got a quest to kill Magtheridon for Trials of the Naaru: Magtheridon's Lair.

This all meant that you had to rep grind to revered with 4 factions just to get the heroic keys, then you had to complete the runs. Back when TK attunement was still implemented, heroics were about 3 times harder than they are now. Dogs in Shattered Halls could very rapidly 2-shot a well-geared tank. Heroic Arcatraz was virtually impossible to do without 2 healers and a mage or druid. Shattered Halls virtually required a paladin tank. Heroic Shadow Labs could chain spawn assassins for 5 minutes straight. It was brutal.

What made it so obnoxious was that to attune 30-35 people you had to run these heroics over and over and over, not to mention running them normal mode to get the rep to get the keys for people. I spent all day long running these damn heroics to the point where I swore I would castrate anyone who even suggested I set foot in Arcatraz. When you're raiding with 3-4 tanks and 7-8 healers, it's your tanks that bear the brunt of the frustration, particularly when you have some less-than-stellar players, it really shows in the old-mode heroics.