EDIT: I discovered the extra action button in the Vorkath fight (arrow icon) is to load up a ballista. Use this after Vorkath flies up. Instead of blasting the arena with ice, he will get shot down like a punk.
The Fort Forinthry season finale walks you through the new Vorkath & Zemouregal boss fight and wraps up the fort story in RuneScape 3 for now. This guide uses a (mostly) full revolution Necromancy setup.
0:00 - Intro/Prerequisites
1:24 - Disabling Places of Power
3:38 - Boss Fight (Story Mode)
5:47 - Regrouping
7:35 - Repairing the Fort
9:18 - CUTSCENE: The Banquet
10:30 - "Interrogating" Zemouregal
12:55 - Commemorative Statue Location (Vorkath Fight)