ngl, I always thought Conrad was sus. Guy was too clean for the position he has.
Dunno exactly how bad he is, but if I was feeling particularly paranoid... He gets the king killed so the isolated and politically-untrained half-breed son gets the job as soon as possible, before he has a feel for politics or any local connections. If Conrad is behind the poisoning, he's already puppeted Cass(?) and that branch of the family to set this whole "emperor come again" thing into play, and can easily become Elis' trusted advisor and rule the kingdom in every way that matters.
Hell, Elis is the only son, once things settle down a little bit and Elis has a son of his own Conrad can outright have Elis killed if he wants and act as regent for the newborn boy king. Blame the poisoning on the queen, play to racial tension, etc. Marry one of the sisters for legitimacy, have a few kids of his own, ensure Elis' kid dies somehow (horse riding accident, stabbed by a beggar, hybrid sickness, etc.) and his own children inherit the empire.
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Unrelated it's hilarious how badly Cassandra wants to be some kinda grand queen Dark Lady Villainess while very clearly losing to her brother's dick and love.