Alf Davies, 94, was one of a few survivors when the German SS ambushed a convoy in the French town of Wormhout.
He described the massacre as "horrific", with many friends killed.
Known as "Ambush Alf", he will be one of the first residents at Cysgod y Gogarth, Llandudno, when it opens, with his memories kept in a time capsule.
Gunner Alf Davies of the 69th Medium Regiment, Royal Artillery was travelling in an allied convoy of three trucks when the ambush happened.
He described "absolute chaos" as the SS started firing from roofs and buildings on 20 British soldiers and 30 French POWs who tried to escape through a river.
Mr Davies said: "Some turned left while me and a couple of mates turned the other way and made our way down the river.
"We later found out that the ones who went the other way were caught and put in a barn with a lot of other British troops.
"The SS called the men out five at a time and shot them in cold blood."
Most of the 50 men died, while Mr Davies and two friends escaped over fields before finding a motorbike and riding to the beaches at Dunkirk.
After leaving by boat, he eventually went on to fight at El Alamein in Egypt, Tripoli in Libya, then Italy, Germany and France. He said "you can never forget" such experiences.
On returning to Llandudno, he was given his nickname and worked for the council.
The great grandfather's memories will be preserved by housing association Cartrefi Conwy who are building the 26 apartments.
Chief executive Andrew Bowden said the time capsule will be a reminder of "the courage and sacrifice of Alf's generation".
The story of a war veteran who narrowly escaped one of World War Two's worst atrocities will be preserved at a new development in Conwy county.