Hafþór is best known for being the world's strongest man, and for his terrifying Game of Thrones role.

Game of Thrones star Hafþór Júlíus Björnsson has proudly announced he's welcomed a baby boy with his wife Kelsey Henson. World's strongest man Hafþór, who played the villainous Mountain on HBO's fantasy hit - penned an emotional tribute to Kelsey and their son, and detailed how the day of the birth had gone. The lad, who's name is yet to be revealed, is their first together, while Hafþór also has daughter Theresa from a previous relationship. The couple who tied the knot in 2018  shared their baby joy back in April, revealing they were having a boy.

Posting a string of snaps of their new family, proud dad Hafþór wrote: "6 am Kelsey woke me up telling me she had been losing her water, I immediately jumped out of bed and said alright let's go, she stayed calm and replied with a laugh and said, no we are not going yet. I then asked to call our midwives to which she replied again no let's just wait for a while.

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"7:15 am we call our friend to come photograph the labour, the midwife and doula. Once the midwife arrived she checked out the progress and we were surprised to learn Kelsey was already 5cm dilated. The midwife asked if she was ready to go to the birthing centre or wanted to try out that bath at home and Kelsey, remaining calm, decided to try the bath at home.

"While I was filling up the bath things started escalating quickly, the contractions were much stronger lasting over a minute and were anywhere from 2-3 min apart. The midwife then said she thought we should head to the clinic and finally Kelsey agreed it was time. 8:40 am. We arrived at the Bjo?rkin, the birthing centre, after a quick 10 min drive. Right away Kelsey knew exactly what she wanted and asked how long until the birthing pool would be filled. The next fifteen minutes Kelsey paced around between contractions waiting for the tub to be filled.

Hafþór is a new dad again. Photo: Instagram.

READ ALSO: Game of Thrones' The Mountain and wife announce they're having a boy

"9:00 am She was in the tub and immediately started to push. Things escalated super quickly. Throughout it all she did amazing. She was in complete control of her mind and body. She had strong opinions on exactly what she wanted and where she wanted to be. I am completely in awe of how strong she is, and so proud of her for everything she did to prepare herself throughout her pregnancy. It cannot be put into words how proud I am and how I feel about this experience.

"11:19 am After an extremely hard 2 hours and 19 mins of pushing our healthy, strong, beautiful boy arrived weighing 3530 grams, 52 cm long. To finish the most beautiful, awe-inspiring labour was the most wonderful phone call to my daughter to introduce her to her new little brother. 5:30pm We arrive back at home both mom and baby doing super well and spent our first night as three together. We have decided on a name and cannot wait to share it with you all soon."