Assassination of Renly Baratheon

The Assassination of Renly Baratheon was an event during the War of the Five Kings. It was the direct result of the failed meeting between claimants to the Iron Throne Stannis Baratheon and Renly Baratheon, his younger brother.

Background
Following the death of Robert Baratheon, his two brothers, Stannis and Renly, claimed the Iron Throne for themselves. They both knew Robert's official heir, Joffrey Baratheon, was not actually Robert's son, but a bastard born of incest between Queen Cersei and her twin brother, Ser Jaime Lannister.

Stannis Baratheon, from his seat in the island of Dragonstone, claimed the throne as Robert's legitimate heir by right of birth, with the sole support of the lords of the Narrow Sea, houses sworn to Dragonstone. Renly Baratheon, from the ancestral seat of House Baratheon, Storm's End, also declared himself King of Westeros with the support of House Tyrell as well as the Stormlords, vassals to House Baratheon.

The Parley
Knowing his numbers were vastly inferior, Stannis set sail to Storm's End to demand the allegiance of his younger brother and the Stormlords. Renly, however, refused to bend the knee, citing the vastly superior numbers of his army and the fact that no lord wanted Stannis as King.

Catelyn Stark, who had been sent by Robb Stark to discuss an alliance between Houses Stark and Baratheon against Lannister and King Joffrey, attempted to make peace between the brothers and remind them of their common foe but was unheeded by both would-be kings. Stannis coldly reminded Catelyn that her husband had supported his claim to the throne, even at the cost of his own life.

Stannis offered to make Renly his heir and reinstate him as Master of Laws in the small council. Stannis gave him until the night to consider his decision, adding that if Renly had not come to him by sunrise, he would be destroyed.

Smuggling the Red Woman
Stannis ordered Ser Davos Seaworth, the 'Onion Knight', to smuggle Melisandre to a seaside cove nearby Renly's camp, and to never mention it ever again. Despite his misgivings, Ser Davos did as ordered and smuggled Melisandre aboard a small rowboat.

The pair found the cove to be barred by a metal gate. However, Melisandre was not cowed by the obstacle and merely removed her robe to reveal her pregnant belly to a horrified Davos. She quickly gave birth to a shadowy, human-like immaterial creature with an appearance similar to that of Stannis that vanished soon after leaving Melisandre's womb.

Death of Renly I
That same night, as Renly's men prepared for the inevitable battle, King Renly held another meeting with Lady Catelyn to discuss the terms of his alliance with Robb inside his tent. Renly offered to accept Robb styling himself King in the North, ruling over all lands north of the Neck, as long as he swore loyalty to him. Catelyn tried another time to convince Renly of making peace with Stannis, only for Renly to retort that treating with Stannis would be like treating with the wind.

After Renly removed his crown and chestplate before a mirror and gave them to Brienne, the shadow born from Melisandre materialized behind Renly, quickly stabbing him through the back with one of its talons. Renly fell, dead, and the shadow vanished, before the eyes of the horrified Brienne and Catelyn.

Hearing Brienne's scream, two of Renly's Kingsguard, Emmon Cuy and Robar Royce, entered the tent. Believing Brienne to be the culprit they attacked her, though the warrior-woman defended herself and slew her two sworn brothers.

Aftermath
Renly's forces largely swear fealty to Stannis when he arrives the next day. House Tyrell lead their troops back to Highgarden. Brienne is blamed for the murder by many and she flees the camp with Catelyn.

In the books
In the A Song of Ice and Fire books there are two assassinations and two 'shadows' carrying them out them. The first one is born in an unknown location and reaches Renly's tent in a similar way, though it slit's Renly's throat beneath his metal gorget instead of stabbing him. Also, Brienne doesn't kill any of Renly's men since Catelyn convinces Ser Robar Royce of Brienne's innocence. Nevertheless, both Ser Emmon and Ser Robar Royce are killed by Ser Loras in a fit of rage, with Loras also believing Brienne to be the culprit. Neither Margaery nor Petyr Baelish are present at Renly's camp during the events; Margaery is at Bitterbridge with the full strenght of House Tyrell while Littlefinger is still at King's Landing.

The second assassination is carried out by a second shadow born after Davos smuggles Melisandre beneath the walls of Storm's End, is sent after Ser Cortnay Penrose, who refused to yield Storm's End to Stannis even after several of the Stormlords had already bent the knee to Stannis. The circumstances of this assassination were transferred to Renly's death in the TV series adaptation.