WoW Heirlooms — Browse All Heirlooms
Track your World of Warcraft heirloom collection. Browse all 134 heirlooms with vendor locations, costs, and upgrade levels. Filter by armor slot and weapon type.
Browse all WoW heirlooms with upgrade levels and acquisition sources. Find heirlooms from vendors, achievements, and special events. Track which heirlooms you own and their upgrade status via Battle.net.
Gronntooth War Horn
Source: Warlords of Draenor Mythic Dungeon Bosses.
The 
Gronntooth War Horn is a 715 item level Heirloom trinket introduced in patch 6.2.3 that drops from bosses in Mythic Dungeons based on your active loot specialization.
Set Your Loot Specialization
Before entering a Mythic Dungeon, set your loot specialization to the role you want the trinket for. Your first drop is guaranteed to match your active loot specialization, so a Holy Paladin with loot spec set to Retribution will receive the Strength trinket instead of the healing one.
Run Mythic Dungeons
Defeat bosses in any Mythic Dungeon — the drop is personal loot, so no other player can take it from you. The drop rate is approximately 5–15%, and players have reported averaging roughly one trinket per 20 boss kills.
Farm Additional Trinkets
Once your first trinket drops for your loot specialization, subsequent drops from the same specialization become random, meaning other Heirloom trinkets such as 
Purified Shard of the Third Moon or 
Judgment of the Naaru can also drop. These cannot be obtained using bonus rolls.
Tips
Gronntooth War Horn can be equipped alongside the Touch of the Void Heirloom trinket simultaneously, giving a fresh level 100 alt two Heirloom trinkets without needing to acquire other gear.
Gronntooth War Horn does not appear in your Heirloom tab until you physically loot it, unlike most Heirlooms.