This talent is presumably based on the Lich King's ability of the same name.
This talent was mentioned in the April Fools' Day patch 9.0.5.0.5 notes: "Defile damage doubled in one dungeon, making it de-better to de-use in De Other Side."
Defile now has additional functionality - If an enemy is inside Defile, it grants 1% Mastery every second, stacking up to 8% Mastery.
Defile is no longer a choice node with [Unholy Pact] and has replaced Pestilent Pustules within the specialization tree (previously located uner [Sudden Doom] and [All Will Serve].
Patch 10.0.0 (2022-10-25): Now a row 6 Unholy talent (was a level 45 Unholy talent). Scourge Strike and Clawing Shadows now hit 7 nearby enemies (was 4).
Now has an additional effect – If any enemies are standing in the Defile, it deals increasing damage every second.
Patch 8.0.1 (2018-07-17): Now requires level 90 (was 100), has a 25 second cooldown (was 30 seconds), and deals Shadow damage (was Shadowfrost). No longer increases your mastery.
Patch 7.2.5 (2017-06-13): Damage increased by 38%.
Patch 7.1.5 (2017-01-10): Damage increased by 20%, and Mastery per stack increased to 300 (was 200).
No longer available to Blood or Frost death knights.
Now has no cost (previously cost 1 Unholy Rune).
Every 1 second, if there are enemies standing in it, Defile now increases your mastery, stacking up to 10 times. No longer increases its damage if there are enemies standing in it.