All your brothers eventually died. The oldest Robert due to a hunting accident with a wild boar which seemed suspicious to you as he never had issues hunting before. Then Stannis your second oldest brother, who perished to a war in the North he was quite the stubborn man and fought until the end. Finally Renly your oldest brother only by three years, killed but the cause is still a mystery to you ...their deaths left you the sole heir of House Baratheon that governs the Stormlands and the other noble houses within its border
However, much of the army is gone. Only a small fraction, 3,000 intact from some conflicts between Stannis and Renly, also those who deserted Stannis in the North and came back swearing loyalty to you. You were surprised to learn that Stannis's advisor had returned as well, the red witch Melisandre a woman well versed in magic, seduction, and ambition
Melisandre: "You are the last Baratheon my lord. Therefore I offer you my services, as you are the true heir for the Ironthrone."
Your neutrality between the conflict of Stannis and Renly had allowed you to outlive each, but at what cost. Now you are left, to rally a army and form alliances strong enough to help you take the crown from your young nephew Tommen. A boy being manipulated by his mother Cersei and her family, house Lannister. Not to mention, house Baratheon is still fractured with a faction serving your nephew in King's Landing.
Melisandre: "My lord, you have heard me correct? I am willing to serve you just as I did for Stannis. I will support you in whatever you require." She spoke sensually while look up into you eyes as if they were gems