Stefán was born on 8 December 1982 in Reykjavík. He began playing chess at the relatively late age of 11, at his elementary school Melaskóli. He showed talent and progressed quickly. His attacking playstyle, strong personality and common refusal to accept draws in equal endgames led to elder members of the Reykjavík chess community giving him the affectionate nickname "Pönkið" ("Punk"). Stefán continued to improve: he won the Icelandic School Chess Championship in 2000, and earned his international master title in 2002, at the age of 19. International master and journalist Björn Thorfinnsson described him as the strongest of his age group. Also in 2002, he won the Reykjavík Chess Championship, a feat he repeated in 2006.
Stefán earned his first grandmaster (GM) norm at the Drammen International, held at the start of 2005, scoring 7/9. He earned his second and third GM norms in 2006. The former came at the April First Saturday GM tournament, which he won, scoring 8/11 (+5–0=6; 2609 PR). He achieved the latter at the 22nd European Club Cup in October, scoring 5/7. Stefán began to compete less regularly after this point as he focused on other pursuits, but he dedicated himself to improving his rating in order to attain the GM title. He was awarded the title by FIDE in October 2011, after reaching a rating of 2500. He was the twelfth Icelander to achieve the GM title.
l competitions for Iceland: five times at the Chess Olympiad (2000, 2nd reserve board; 2002, reserve board; 2004, 2nd reserve board; 2006, reserve board; 2008, board 4) and four times at the European Team Chess Championship (2001, board 4; 2003, board 3; 2005, board 2; 2007, board 4). His overall Olympiad score was 23/41 (+18–13=10), and his overall European Team Championship score was 16½/32 (+11–10=11). Iceland's best Olympiad result with Stefán on the team was 22nd in 2002, and their best European Team Championship result was 20th in 2001 and 2007.
Stefán achieved a peak rating of 2503 in March 2014, and was the No. 10 ranked Icelandic player as of November 2017 with a rating of 2447.
Stefán died on 28 February 2018, aged 35. The cause of death was not publicly announced. Vísir.is reported that Stefán's death was greatly mourned by the Icelandic chess community; Björn Þorfinnsson wrote that it will take a long time for Icelandic chess to recover.
Wikipedia article: Stefán Kristjánsson