I was stuck on this part myself, The first time I charged the fort and was knocked back. The 2nd time, i stood from a distance targeting and saying things like "you need to leave now" etc.. This advice I got from an in-game necro. After some time passed, I suspect the same as if I had done nothing, they just leave, and you get a red emote assuming you have failed. When you return to the first portal again, an undead looking guy will stand where the head once stood. Hailing him spawns one of the guys you spoke with earlier who will attack once he paths. He will root/stun you but every so often goes to a "Venerable" status and turns into the undead version of himself, this is when he will take damage. Kill him for the win.
Also, I'd like to note on the bard, you don't simply snare him once, you must repeat this until the process continues. I had to snare him about 5-6 times. You will get an e-mote saying he "stumbles on a song" or etc. Once the female spawns you need to send your pet in, and fear her to make her attack the bard. You must rinse and repeat this as well. After a few goes, it will complete the step and you can move on.