There are a number of hints to the player as to who the assassin is, mostly through eliminating who can't have done it by reading dialogue in cutscenes and working out what they were up to. There are three pieces of 'real' evidence: dagger, parchment from bird, black cloth. The fact that the dagger had Branwyn's crest implies that either someone had an existing dagger (or it was one of Branwyn/Taliesen), or that someone had crafted a dagger to lay the blame on the merceneries.
Now, the people in the castle at the time of the murder were: Chancellor, Geraint, Iseult, Ruadh, Midir, Rhiannon, and you, that Maeldun knows of. Branwyn, Finegas and Amairgen left the castle, and since they said they'd travel together, you know none of them could've remained in the castle. That doesn't mean to say that none of them could've gone back to murder the king, but it's much less likely.
- Ashe was never allowed in the castle, Changeling was too afraid of leaving Thierna na Oge, and Niameh is too old and probably at home at the time of the murder. Also, the sorceror's apprentice probably wouldn't kill the king since she was bound by the Pact, and if she did, she would probably use magic instead.
- There is no evidence pointing to Finegas or Amairgen, and in fact both are rather helpful to you. Both don't possess means of magic or stealth to re-enter the castle after having left it, therefore they are unlikely candidates.
- Branwyn and Taliesen both did not like Vortigern due to past conflict but both have each other as an alibi. Also, Branwyn is a warrior, not an assassin, and though Taliesen knows some magic, he mentions that doesn't know enough to do something complicated, say, for instance, teleport into the castle. Therefore they are ruled out.
From the cutscenes and dialogue, these are more hints you get:
- Chancellor/Geraint: Geraint mentions that the chancellor was doing something dishonourable, implying that he knew where the chancellor was at the time of the murder.
- Chancellor/Midir: Chancellor mentions that Midir's behaviour was disgraceful, and Midir tells you he was having a drink with 'a friend' and got drunk. Note, if you show the black cloth to Midir, he will tell you that someone bought a cloak from him recently. Also note that Midir and Cailte are friends from the cutscene between the two. Midir is an unlikely candidate for a murder given that he was snoring fast asleep during the whole event.
- Jeweller/Ruadh: Discuss 'a deal'. You know that the girdle is in the jeweller's shop, implying that she somehow stole your belt... most likely, Ruadh had stolen it, which he later admits to. (If you befriend Ruadh, he'll mention offhand that the Jeweller and Paradur are lovers - this is what Geraint was referring to as 'disgraceful' behaviour by the chancellor. This was subtle, but note that the two were quite friendly to each other in the library, that Iseult stuck up for the chancellor by admonishing Rhiannon in the dinner scene, and that she's later seen visiting at the castle). This effectively rules out Iseult, Ruadh, and Paradur from commiting the crime.
This leaves three people left: Rhiannon, Cailte, Geraint.
- Rhiannon is not the murderer primarily because she was already in the throneroom by the time the guards took you there. The assassin wouldn't have been able to get into the throneroom without being seen, therefore the assassin can't be Rhiannon (also because it's highly unlikely that she'd murder her own father at such a time like this)
- I suspect that most players might've thought that Geraint was the murderer (or Paradur). Geraint isn't the murderer mainly because evidence doesn't truly suggest that he is, only suggestions by characters such as Taliesen. Also note that the guard at the gate castle says that Geraint stays with the soldiers in the barracks each evening, effectively giving him an alibi. Character-wise, Geraint is too loyal and honourable to murder his own king.
This leaves Cailte.
- Talking to townspeople about the king's death: All of them will acknowledge that the king was murdered except Cailte. Also, Cailte seems quite intent on having Geraint as king, and calls King Vortigern weak and feeble. If you show him Branwyn's dagger, he will mention that it's not interesting, and that he could craft one easily within a week. Cailte seems in denial about the whole situation. In a cutscene between him and the sorceror's apprentice, he insists she find out where Maeldun the murderer is - this may throw some people off from thinking it's Cailte, but note that Cailte insists that Maeldun is the murderer - his intention is to see whether the apprentice can discern the truth, but more the fact that he wants Maeldun scapegoated so there's less chance evidence will pop up against him.
- The third piece of evidence - the note: Implies there's political intrigue going on. Cailte isn't particularly in a position of power, but the wizard is powerful, and can convince Cailte to his cause and get him to do his bidding.
So in summary, while Iseult and Paradur were spending time together, Ruadh was stealing Maeldun's girdle, and Geraint spending time with his guards, Cailte got Midir drunk, then carried out the murder. He had made a dagger beforehand to ensure the blame was laid on the mercenaries, and had bought a cloak from Midir to ensure that he was not easily recognised. He had enough motive, despising Vortigern for losing to Whiteblade years ago, and he certainly had opportunity to carry out the crime.
