The spawns can be buggy so they may not follow this order before the portal spawns. Also the portal appeard directly below the Shadow Cyst and the dot won't always be visible on the radar because the Cyst's dot is on top.
1. Twisted Shadow, Shadow Nightmare 2. Void Knight, Wretched, Twisted Shadow
Inside the Shadow Spire Umbral Soldier, Wretched, Void Knight, Penumbral Soldier, Twisted Shadow, Captain Faalx
Climb the stairs to the top of the tower, when Captain Faalx is killed a portal to the Spires Head opens. It can be difficult to see, it's a swirling black mist on the ceiling and must be double clicked to enter.
The Shadow Cyst drops 10 keys (Boney Lump of Flesh) to the Fleshy Trove. The pickup timer is 13 days. The Fleshy Trove contains the Shield of Isin Dule and a ??.
You give Bretself the Translator Fleshy Lump.
Bretself the Translator tells you, "Ugh! Another of these lumps? Even worse, this ones still squirming! Still... this is fascinating."
Bretself the Translator tells you, "This seems to be orders to the Shadows under Isin Dule's command. Oh my, he raised a Shadow Spire? He's ordering his forces to stop some High Archon of the Raven Hand. Here, take a look."
Bretself the Translator gives you Isin Dule's Orders.