Cressen is the Maester of Dragonstone. He dies trying to trick Melisandre into sharing a poisoned cup of wine with him.
Cressen is a maester of the Citadel, a man of learning assigned to a castle to advise its inhabitants in matters of science and medicine. Cressen has served Lord Stannis Baratheon at Dragonstone for many years.
Cressen is worried about the influence that the red priestess Melisandre has over Stannis, and is horrified when she gets him to renounce the Faith of the Seven. When she has the carved wooden statues of the Seven burned on the beach, he speaks out against doing this. He says that all of them were named in the light of the Seven, and they risk the wrath of the Gods. Cressen tries to talk to Davos about going against her, but Davos refuses, saying that right now is not an opportune time to act. The maester decides that if Stannis can't be talked into putting Melisandre aside she must be killed, before she leads him into a war he cannot win.
Later Cressen sits at the Painted Table, at a council meeting with Stannis, Davos, and Melisandre. When he puts poison in a cup of wine, Davos silently urges him to stop, but he does not. Walking around the table, Maester Cressen says that they should put their differences aside and asks Melisandre to drink a toast with him. He drinks from the cup first and hands it to her. She watches him and sees his nose start to bleed. She knows it's poison, but with a smile she drinks the rest of the wine anyway. As he dies almost immediately, she stands over him, unaffected by the poison.
When Shireen Baratheon is teaching Gilly how to read, she mentions to her and Samwell Tarly that Cressen taught her how to read when she was three years old.
When Samwell tells Archmaester Ebrose that Shireen Baratheon was cured of her greyscale to try and persuade the Archmaester to allow him to try and help Jorah Mormont, Ebrose responds that Cressen accomplished this when Shireen was very young and the disease had not spread so much.
Maester to Stannis Baratheon, Cressen grew alarmed by Melisandre's increasing influence on Stannis and the king's rejection of the Faith of the Seven. When Cressen tried to poison the Red priestess, he succumbed instead.