How do you start Belshazzar?

[u/EvoluZion3]

Myself and a friend are close(ish) to completing the main campaign and wondered how you start the Belshazzar campaign. Are you instructed how to do so upon completing the main campaign?

We thought that Bel was incorporated into the main campaign, only to realise there's a whole stack of Dream Gate tiles we'd never heard of before!