The Astral Walker is a title awarded when you complete the achievement  Heroic: Observed by defeating Algalon the Observer on heroic difficulty.

The title appears after your character's name:

"<name>" the Astral Walker

v • d • e Player character titles Arena Vanquisher · Gladiator (Merciless, Vengeful, Brutal, Deadly, Furious, Relentless, Wrathful) · Duelist · Rival · Challenger · Flawless Victor · Arena Master Rated Battlegrounds Warbound · Hero of the Alliance · Veteran of the Alliance · Warbringer · Hero of the Horde · Veteran of the Horde Rated Battleground Rating Private · Corporal · Sergeant · Master Sergeant · Sergeant Major · Knight · Knight-Lieutenant · Knight-Captain · Knight-Champion · Lieutenant Commander · Commander · Marshal · Field Marshal · Grand Marshal Rated Battleground Rating Scout · Grunt · Sergeant · Senior Sergeant · First Sergeant · Stone Guard · Blood Guard · Legionnaire · Centurion · Champion · Lieutenant General · General · Warlord · High Warlord Battlegrounds Conqueror · Justicar · Battlemaster Exploration the Explorer Feats of Strength PvE Bloodsail Admiral · Scarab Lord · Champion of the Naaru · Hand of A'dal · Conqueror of Naxxramas · Obsidian Slayer · the Magic Seeker · the Celestial Defender · Death's Demise · Grand Crusader Arena Gladiator (Merciless, Vengeful, Brutal, Deadly, Furious, Relentless, Wrathful) · Vanquisher Honor System Private · Corporal · Sergeant · Master Sergeant · Sergeant Major · Knight · Knight-Lieutenant · Knight-Captain · Knight-Champion · Lieutenant Commander · Commander · Marshal · Field Marshal · Grand Marshal Honor System Scout · Grunt · Sergeant · Senior Sergeant · First Sergeant · Stone Guard · Blood Guard · Legionnaire · Centurion · Champion · Lieutenant General · General · Warlord · High Warlord Professions Chef  · Salty PvE Jenkins · the Patient · Champion of the Frozen Wastes · the Undying · of the Nightfall · the Immortal · Twilight Vanquisher · Champion of Ulduar · Conqueror of Ulduar · Starcaller · Herald of the Titans · the Astral Walker · Argent Defender · the Kingslayer · Bane of the Fallen King · The Light of Dawn · Blackwing's Bane · Dragonslayer · of the Four Winds · Defender of a Shattered World PvP of the Alliance · of the Horde · the Bloodthristy Quests the Seeker · Loremaster Reputation the Exalted · Ambassador · Guardian of Cenarius · the Argent Champion · the Diplomat · of the Shattered Sun · the Insane · of the Ashen Verdict World Events Elder · the Love Fool · the Noble · Matron · Patron · Flame Keeper · Flame Warden · Brewmaster · the Hallowed · the Pilgrim · the Merrymaker Argent Tournament of Darnassus · of the Exodar · of Gnomeregan · of Ironforge · of Stormwind · of Orgrimmar · of Sen'jin · of Silvermoon · of Thunder Bluff · of the Undercity · Crusader