Okay I found my old post. It was the transfer cb exploit.
Release or force-vassalize a nation your target would be domineering towards.
Take your vassal and hug it. Tell it how special it is. Slap your dynasty on its throne and check heir claim strength.
If it's medium or high, enjoy sitting around fiddling your thumbs. You need low.
Thankfully putting your ruler on their chair lowered legitimacy to 20. Once their ruler dies their heir is bound to have a weak claim.
If you get a weak claim from the start, rejoice! Now kick that bastard child out of your house.
Claim throne immediately. Wait until your enemy vassalizes your former vassal.
Entertain yourself by staring out the window and contemplating existence. Not yours, but rather the existence of existence. It's essential for the exploit to work, trust me, I exist.
Once your former vassal found a new overlord you can declare on him with claim throne cb. Overlord gets called in (obviously) and the cb gets transferred to him.
Congratulations! You can force union on your former vassal's overlord!
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