Sentinel Tree

Phase 3 with only Shadow Wardens recruited

Phase 4 with food shrubs and ancient treants

It is wood. It is in the Firelands. Why is it not on fire? RAGNAROS INFURIATED!

The Sentinel Tree is a tree with roots in Azeroth, but a canopy in the Molten Front section of the Firelands to provide some protection to the Guardians of Hyjal as they counterattack the Firelands Invasion.

Malfurion Stormrage plants the original seed in a small cave after the player completes  [85] Through the Gates of Hell. As the player spends Marks of the World Tree to recruit more factions to aid in the Firelands Invasion the Sentinel Tree grows from a tiny seed to a respectably large tree whose roots enfold the original cave. At the same time the area surrounding the tree transforms from a black pumice wasteland to a slightly more inviting expanse of patchy green undergrowth.

Some of the greenery and scenery is linked to specific phase 4 vendors like Varlan Highbough and Ayla Shadowstorm.

General Taldris Moonfall and Malfurion Stormrage have conversation options that briefly explain the Sentinel Tree.