The newly discovered chess piece was bought for £5 (€5.59) in Edinburgh in the 1960s.
ADVERTISEMENTA formerly-lost medieval chess piece could sell for over €1 million at a London auction on Tuesday.
An Edinburgh antique dealer bought the piece for £5 (€5.59) in 1964 and it was passed down through his family.
The "warder" is part of the most famous collection of medieval chess pieces.
Discovered in 1831 on the Isle of Lewis, the Lewis Chessmen comprised 93 pieces of which 82 are now in the British Museum in London and 11 in the National Museum of Scotland's collection. The majority of the pieces are carved from walrus ivory.
The location of five of the chess pieces remained unknown, and the newly discovered Lewis "warder" is the first discovery of a missing piece of the collection.
One of the family members felt the small chess piece was special and held "magical significance", a family spokesperson told Sotheby's.
The chess pieces have inspired many generations and long been recognised as an "important symbol of European civilisation".
"This is one of the most exciting and personal rediscoveries to have been made during my career," Alexander Kader, Sotheby's expert on European sculpture and works of art, said in a statement in June.
Scholars believe they are Norwegian, potentially from Trondheim.
This will be the first Lewis chess piece to be sold at an auction and it's expected to sell for between £600,000 and 1,000,000 (€669,945 and 1,116,575).