The Unbreakable Vow is a spell where a witch or wizard makes an oath to another, as the one made between Narcissa and Snape.

Pattern is available in 3 different weight yarns -

Worsted:
Worsted weight yarn (4) - about 135 yards
US 7/ 4.5mm 16” circular needles for brim
US 8/ 5mm 16” circular needles/ dpns

Bulky:
Bulky weight yarn (5) - at least 100 yards
Ie. Malabrigo Chunky, Lion Brand Color Made Easy
US 9/ 5.5mm 16” circular needles for brim
US 10.5/ 6.5mm 16” circular needles/ dpns

Super bulky:
Super bulky weight yarn (6) - about 87 yards
US 11/ 8mm 16” circular needles for brim
US 13/ 9mm 16” circular needles/ dpns for adult S/M
US 15/ 10mm 16” circular needles/ dpns for adult L

Special stitches explained in pattern with photo tutorial.