OTL Henry Trastamara managed to kill Peter II of Castile at the Battle of Montiel.
What effects if Henry is the one who dies and with it his claim to the throne.
It accomplishes nothing.
Peter has lost the war before Montiel, his armies are made of exhausted bottom-of-the-barrel militia badly put together and the only thing keeping him in life support is the 7000 or so men that the King of Granada gave him and that were crushed at Montiel. The Enriquistas are in control of Old Castile, León, Toledo and Cordoba, in other words nearly everything that is worth controlling in the kingdom, and the English have left him.
Bear in mind that Peter didn't die in battle. Following the destruction of his army he took refuge in the castle, and while under siege he contacted DuGuesclin's French mercenaries and tried to buy them into letting him escape. DuGuesclin took the money and handed him to Henry instead, who at this point could have done anything to him. That Henry consented to fight a duel to the death with Peter was practically a gift to him.
So winning the battle is unlikely, winning the war impossible, and the best Peter can aspire at this point if he *does* win the battle is to find a way to retire to Portugal by way of Andalusia with what troops he has left and try to take the kingdom back with foreign support like he did two years earlier from Guyenne.
By the way, even if a cosmic allignment of some sort killed Henry at Montiel his supporters would simply crown his 11-year old son and after him there is still Henry's brother Tello Lord of Aguilar and Biscay and Tello's adult son Juan Téllez who IOTL would go onto die at Aljubarrota.
Oh, and finally, Peter did have sons. They just died in prison after Henry's victory. John of Gaunt has not a chance in Hell to be king of Castile.