With some careful use of the world activation mechanics, it's possible to set your adventurer up as the monarch of a dwarven civilization (without modding or dfhack).
Step 1: Generate a world with at least one dead dwarven civ
(https://i.imgur.com/CK9Qmkc.png)
For this proof of concept, I made a high-savagery pocket region filled with megabeasts. They quickly wiped out every civilized site.
Step 2: Create a fortress
(https://i.imgur.com/06EAQOf.png)
You should see the "your civ is dead or dying" message before embark.
Step 3: Wait for a random dwarf to become king/queen
(https://i.imgur.com/V5unHGj.png)
It should only take a few moments after you unpause. If this doesn't happen, you might need to gen a new world and try again.
Step 4: Retire the fortress and create an adventurer
(https://i.imgur.com/ApB0z8v.png)
Since this is a dead world, my only option here is Human Outsider, but any kind of adventurer should work in theory.
Step 5: Retire the adventurer at your fortress, then unretire the fortress
(https://i.imgur.com/Z45vbBd.png)
You should now see both your current monarch and your adventurer in the unit list.
Step 6: Arrange for an Unfortunate Accident
(https://i.imgur.com/G5bGnlI.png)
Step 7: Profit
(https://i.imgur.com/nGXlrIh.png)
Some notes:
- This means it is possible to have your civ ruled by say, an immortal vampire necromancer elephant woman queen. Or a gorlak.
- Theoretically you could do this without a dead civ, it'd just be much more difficult. You'd need to assassinate everyone in the royal line of succession before you.
- To have your adventurer be a full citizen of a fort, you need to become a militia-dwarf before retiring (talk to the militia commander)
- You can unretire the adventurer again at any time and go off on little royal ventures