Currently event is not implemented.
When you kill the Grand Warlock Nethekurse, you climb up and see a long corridor, stepping into this area - Event starts.
f) If players come up at the ramp, Shattered Hand Scout (GUID npc) shall yell Invaders have breeched the defenses (Sound ID: 10285), run along the hallway to Blood Guard Porung Shattered Hand Bloodguard and despawn there
Source http://www.youtube.com/watch?v=4VFpnpn4_ng
Start after 1:01
Then the 2x Shattered Hand Archers shall shoot burning arrows (most likely spell 37035) along the hallway if players stand in line of sight. (source: You can duck into a cubby on the side to take a very short breather. This de-aggros the archers [...]) Those arrows should hit the ground, explode with AOE damage and leave burning spots on the ground. (source)
Shattered Hand Zealots should start to run at the players. You can completely observe this event here and here.
For the information, links and videos to be commended!
FeenixServerProject/Archangel_2.4.3_Bugtracker#2569