Welcome to the Demonology Warlock guide written by Sjeletyven. You know who I am.
Demonology is a fairly simple “builder/spender” with some cooldown and resource management thrown in.
What has changed
Not many changes happened from patch 9.0.5 to 9.1 in terms of rotation or covenant choice. Dreamweaver is rising up to be the best soulbind assuming you don't have to move out of your Field of Blossoms.
Necrolord is also highly competitive in terms of output, however night fae is more flexible for playing multiple specs and their soul shape is a godsend in many situations.
Changes from BFA to Shadowlands
The biggest change from BFA to Shadowlands is that they reworked how your level 50 talent, Demonic Consumption works. It now drains a percentage of all your demons' health, instead of consuming all of your imps to empower your Summon Demonic Tyrant.
Tyrant also received the effect of the Baleful Invocation Azerite Trait that refills all of your shards when you summon your Tyrant. They made our rotation smoother and reworked the Explosive Potential into a legendary. There was also an undocumented change to Demonology imps that they last for around 40 seconds when you are not in combat, making it easier to carry over resources from pull to pull in dungeons.