so darinius' rise to power involves a lot of... riddles, for some reason?
as i said earlier, he's raised by calpurnia, a priestess of dumat, as her own son, and naturally he shows a bunch of magical talent early and he can do it better than experienced dreamers at the age of ten blah blah blah. interestingly he's said to have been able to charm animals and to use birds or cat as "agents and spies", so effectively that many believed he could tell the future. (darinius is quoted without any real context as having said, "the wise are wary of prophecy: to seek knowledge of the future is to grip a sword by the blade." it's worth noting that his supposed biological mother, livia, was a high priestess of razikale, a role which can also be referred to as the "augur of mystery". i don't have a point here, i just wonder if you could make the argument that there was maybe more to the fortune-telling rumours if you felt like it.)
when darinius was 19, the high priest of dumat lay dying. he summoned all the senior acolytes and of course also darinius even though he was only a novice. his successor, he said, would be the one who could bring him "that which has no legs yet must dance, has no lungs but must breathe, and has no life yet lives and dies". while the other acolytes grumbled that the man had lost his wits, darinius lit a brazier and brought it to the high priest, understanding that what he meant was fire. this apparently qualified him to be the next high priest.
later, the high king of neromenian died without an heir. a successor had to be chosen from among the dreamers, so the palace summoned the high priests of the "three patron gods" of neromenian: dumat, toth, and lusacan. (silence, fire, and night, respectively. i think different kingdoms having different patron gods is super interesting!! i guess we can assume razikale was one of tevinter's.) the three were told they had until sunrise to "tie an egg in a knot" and place it on a pedestal. the correct solution would reveal the high king's crown. while the other two went straight into trances to search the fade for an answer, danarius smashed the egg, soaked a cloth in it, and tied the cloth in the knot, which apparently qualified him to be high king of neromenian.
(this kind of deeply silly story reminds me of two anecdotes: alexander the great and the gordian knot, and the egg of columbus (or of brunelleschi, which was the version of the story i heard first). the wikipedia page for the latter also specifically mentions that someone is asked to tie an egg in a knot in the kalevala, which i'm not personally familiar with. my point is that pulling off a famously "impossible" task by taking a shockingly straightforward approach that others wouldn't have considered is general shorthand for portraying someone as a brilliant visionary who changed the world because he personally was that smart and Not Like The Others.)
on the evening of darinius' coronation, calpurnia reveals the truth to him that he was found on the seashore, and shows him the broken ring. (i'm not really sure why calpurnia is supposed to have kept this secret, but it's maybe sort of implied that it relates to her practices as a priestess of the dragon of silence? the wiki uses the phrase "history would remember her for her silence" but i can't figure out where on earth it got that because i'm looking at the page it claims to reference. it's kind of wild that it was fine for her as a priestess to raise the kid in the first place let alone claim him as her own for so long, but i guess that as the priestly class here seems to be synonymous with the dreamer/mage lineages, thereās no sense that they shouldnāt have families and they were probably actually encouraged to have kids?)
anyway, darinius' new war chief helpfully recognises tevinter's signet ring and they apparently piece together that he is therefore the lost son of tevinter's usurped queen. darinius is now in a tricky situation, because of course he is obliged to avenge his birth mother's death and the gods would put a curse on his lineage if he didn't, so he just has to conquer tevinter, isn't life so hard. however, it might be a bit difficult, as minrathous is well-fortified and hasn't had diplomatic contact with neromenian or qarinus either in decades because they refuse to recognise tarsian as a king. not because he murdered his sister, but because heās not a mage.
that night, darinius dreams that he is crossing a river in a small ferry, piloted by a man whose face is in shadow. when he reaches the other side, he looks back and sees his own face; he was the ferryman. he takes this as a sign from dumat (sure, i guess), and sends a bunch of messages & gifts to his uncle tarsian, who has no idea who he is except that heās the high king of neromenian, saying that neromenian would love to be friends again. tarsian is flattered and lets darinius and his honour guard of seven into his palace in minrathous. darinius then immediately uses a spell to keep tarsian's guards from entering and challenges tarsian to a duel. (world of thedas says that he challenges him and then when the spell dropped and the guards presumably poured in, tarsian simply āwas deadā. i wonder if weāre conveniently skipping over something like how much of an honourable duel it could actually be when darinius brought 7 friends.) anyway darinius kills his uncle and takes the other half of tevinter's seal from his body, apparently managing to reforge it then and there. neromenian and tevinter are then united under his rule.
so this is where we really come to the crux of the matter where darinius has to be the long-lost prince of tevinter not only because it makes him of proper dreamer lineage but also because if he isn't, then he simply lied and tricked and murdered his way onto tevinter's throne. which he totally did. but it's fine because his uncle betrayed him first. apparently.
i frankly don't quite get what the ferryman vision told him. i guess that he had to make the first move against his uncle? nobody else but you can ferry you onwards? i don't know. regardless, this is a major deal for him to the point he's known as "darinius the ferryman." in art, his staff is depicted as a ferryman's pole. the seal of the imperial archon would forevermore be the image of a hooded ferryman, a ring worn on the third finger of the right hand, which is traditionally destroyed upon the archon's death and then a new one made for the next one. however, they're powerful magical items and supposedly sometimes a forgery is ritually destroyed instead. in dragon age 2 you can buy supposedly the ring of archon hadarius.