Onslaught Harbor is the base of operations for the Scarlet Onslaught in Icecrown within Northrend. It is located just off the western coast of the large glacier. The harbor itself is built on a small island. A bridge connects the harbor to a larger island, upon which sits the Crimson Cathedral, in which the recently resurrected Archbishop Landgren resides, surrounded by Onslaught Raven Bishops.
According to Lord-Commander Arete only the battle-hardened Scarlet Crusaders who survived the Forsaken attack on New Hearthglen reside here. That, combined with the gryphons they took, makes the base very hard to conquer. After the player receives a note from Barean Westwind, Lord-Commander Arete sends the player to get information about the Admiral's whereabouts. However, after beating the Scarlet Captains, they only say that Archbishop Landgren might have the information.
Lord-Commander Arete, confident that the Archbishop knows where the Grand Admiral is, sends the player to eliminate the Archbishop. Arete interrogates his soul and finds out that the Grand-Admiral is hiding in a heavily guarded cave behind the cathedral, known as the Hidden Hollow and sends the player to deal with him. It is then revealed that the Grand Admiral is in fact the dreadlord Mal'Ganis.
- As Icecrown is surrounded on all sides by high, sheer cliffs, it is unknown how the Scarlet Onslaught planned to actually assault the glacier from Onslaught Harbor. The closest place to the harbor that their ships could dock on Northrend's mainland would be the coast of Borean Tundra.
- Patch 3.0.2 (2008-10-14): Added.