Makh
In Valeria how about a sword which could control the weather and summon Thunder also the user could even summon it just like thor's weapon storm breaker.
God's, a higher being or a fallen warrior who was killed in battle and on his last breath he threw the sword in fabric of time and space to find a worthy successor who will continue his legacy.
Makh:and where will that come from?
Actually this brings him even passively to Targaryen loyalist side ( cause Martells are still for Targaryen even though in secret, unless author changes something ). This can lower opinion of Robert about young Lord and probably bring suspicion if we consider his white hair. What else can be added as good army tho? I suggest some Arachnomorphs, as vale has a lot of caves and dark places and they can be used for good defence ambush attacks in vale and they are just badass spiders. Also, if we count all monsters from bestiary of Witcher as beasts, archespore can be added as a natural way to get rid of a Reach and have control over their food production. Iron islands can get siren and a lot of drowners. Lannisterâs can get either chort problem or some Ekima as they have a lot of mountains there( not as much as vale but still enough ). Also, will necrophages will make appearance? In Westeros death is natural and there should be enough cases with no proper burial or bad burial or no burial at all after all this attacks.