Overheard verbal discussions close to Viv as best I can remember and paraphrasing the details:

In the later series Alastor meets Emily 'coincidentally' in the hotel when she portals in. After he heard about her doing it before, he somehow figures out when she will again and makes sure Charlie won't be there. He offers to 'take a message' but charms Emily and starts subtly influencing her. She becomes enamoured with Alastor over time and eventually is willingly doing his bidding. By the end she has basically become evil, but is still naive so doesn't realise it, thinking Alastor wants to unite Heaven and Hell to make shit better, like the crap she was so pissy about in season 1.

Final climactic scenes are a simultaneous Hell-wide turf war and plan to destroy Heaven entirely initiated by Alastor. He has spent the series moving all his chess pieces into place for the final merciless attack using Emily as an insider to drop Heaven's defences.

He wins easily in Hell, having killed or undermined many overlords already, finally achieving his dream of becoming absolute ruler there. We find out he planned that all along ever since he was human, having some prior insight into the workings of Hell, (when he made a deal with Rosie he knew exactly who he was talking to).

Now having access to the full power of Hell he attacks Heaven. Charlie who sorta trusted Alastor to this point realises she dun goof'd. Her father no longer there to protect or save her, she basically 'grows up' in that moment having to make a choice between her dream and saving Heaven.

She attacks Alastor in her now uber-powerful demon form depowering him back to his previous level, but in a final contingent act of treachery he manages to kill Charlie.

After the dust settles we see Alastor welcome the cast down Emily as his Queen of a now empty Hell, because Charlie's sacrifice has redeemed everyone else and they are now with her happy and singing in Heaven.
>yfw Charlie is basically 'Hell Jesus'
ffs Viv.