Once you complete the quest to consecrate the temple in the ruins of the pagan town of Galibornium for Judge Welfhar, a few priests appear there that are looking for the stone.

Thanks again mate. Also i didn't complete yet the quest that Ihman Ab Sahab gives you. He wants 3 statues & i only managed to find one, near the tree of death. Where are the other two?

The Statuette of Michael is in a stash in the Pagan Temple of Galibornium (not far from the priest that asks you for the Foundation Stone) and the Statuette of Gabriel is in the northern church in NE Alvaron (near Pontifex).

I'm unable to progress in this quest, I wonder if I broke it? Here's how I did it:

1. In the beginning of Act 3, I went to see the King (didn't know he was dead)

2. I examined his corpse and my journal updated with the information.

3. I talked to everyone in the castle, including the real murderer, who had a scar in his face. He was pissed of, but that was it.

4. Shortly afterwards, I went to the Citadel and received the quest from Grandmaster Cristoffer, along with a letter to Father Gustapho.

5. I talked to Father Gustapho and gave him the letter. He also told me what I already knew: Rodrigo killed the King.

6. I rushed back to the castle and talked to the him, but there are no more dialogue options.

Also talked to Gregorius for permission to arrest him, but there is no such option.
Looked for the Grand Duke and the Sheriff, but neither of them say anything about it.

Am I missing something here? I want that promotion ;)

Maybe you are missing one more piece of evidence for Gregorius to issue the warrant for Rodrigo de Velacor. You also need a proof from Trisstan in NW Alvaron which you will obtain after you have completed his quest to hide his granddaughter Adeleida.

Yes, that's it! Thanks, mate :)

Because finding the statuettes of Michael and Gabriel is a little eye-straining, I'm providing the exact locations.

IMPORTANT:
I found that, if you don't have the testament of Father Bartholomeus in your inventory(name?- it's the one you get in the ecclesiastical tomb and reveals the locations of the items), the caches at Tree of Death and Galibornium will display the message «this chest is locked».