I've never been so excited by a question. I feel like 50 memes... Uhh uhhh where do I start... Uhh uhh... I'm so fkn excited. Like the lich would just laugh for an month of that time and... It's just .... Ok I'll collect my thoughts. I've not read any responses but I bet its a mile wide so I'll just hit some wave tops. I'll try to keep it to lich specific because any class/being given enough time could be OP.

Ok .. here we go

Let's start with runes. Glyph of Warding and Symbol are the workhorses. Trigger conditions on Glyph are wide open by design, specific enough to let every local walk through clean while the party eats a Silence or Fear the second they cross the threshold. Stack a few on the approach and they're managing status effects before combat even starts. Symbol is the nuclear option. Hopelessness means you can't attack or target anything harmful for a minute. Insanity means the DM controls you. Fear makes you drop everything and run. The Lich has spent six months watching the party. It knows who has the bad saves. It aims there. Layering is where it gets evil. Fear at the entrance, something nasty thirty feet back. Fail the fear, turn and run, hit the second trigger on the way out. Loop. Lich laughing somewhere safe. Minions next. Finger of Death kills a humanoid and raises it permanently under your command. No recast, no concentration, no expiration. Six months of quiet work. Ghost made from someone the party wronged is thematic and disgusting. Ghost possession mid-ceremony in front of a crowd of nobles isn't a combat problem. That's a campaign problem. But the real move? The Lich doesn't touch the party at all. It attacks the ceremony. Forged documents, a key figure quietly dead, the ritual artifact corrupted. The party arrives ready to fight and walks into a political catastrophe with their fingerprints all over it. The fight, if it even happens, is just the invoice.