There's a little more I'd say about this trophy.
You can get several under one save file, Which I wish I knew. But it does mean I'm not wasting time on my Mage run.
Firstly, Save at the choosing of a new high king. For these trophies you want at least one faction at maximum at "Nice", all the others can be friendly. Duncas High King
Select Duncas Kill the Final boss and Kill Constantine and you'll get.
Now it's important TO SAVE BEFORE THE DROP DOWN TO THE FINAL BOSS, at the sanctuary itself.
Reload the sanctuary save and do this trophy by opting to "Bind with Constantin"
Now when the credits role and you get this trophy. Reload your high king save Ullen as High King
Select Ullen and then go back through and kill Constantin
Reload your High King saveDeirde as High Queen
Killing Constantin with Deirde as High Queen will unlock
EDIT: For the solution to work, when getting the support of the pretenders, YOU MUST gain their support by doing the side quests for all three.
I've heard that not doing so may actually block some trophies.
EDIT 2: Confirmation, You have to do their side quests to gain their trust. To do all four endings in one save file, You need to do all three side quests (one for each pretender). If you do not have their support, they CANNOT become High Ruler.
EDIT 3: One faction must be at Suspicious or Nice to gain these tre high king trophies. If they are all friendly, the Gold Trophy unlocks instead.