The Ending Winter (冬の終わり Fuyu no Owari lit. End Winter) was a calamity caused by the combined powers of the Legendary Weapons of Elibe.
The Eight Heroes used the weapons against the dragons in The Scouring, only to find that the power of the weapons was so great, that the laws of physics were horribly distorted. For nature to correct this imbalance, the Ending Winter occurred. The Ending Winter sapped much of the potential magic energy out of the universe, and both dragons and the legendary weapons lost much of their power as a result.
Fearing that the weapons' power may inadvertently trigger the apocalypse, the eight heroes hid the legendary weapons of Elibe away in secret locations; all heavily guarded by traps and elemental hazards, to make sure they would never be used.
Another powerful weapon was the Sword of Seals, which was used to seal away the Demon Dragon by Hartmut. Thinking that the most powerful weapon would be once again wielded (which it was, by Roy), Hartmut hid the sword in the Shrine of Seals, which also served as Bramimond's shelter as he maintained the seal over the weapons, so that no mortals could touch them.
Centuries later, Roland's sword Durandal and Durbans's axe Armads were unsealed during the events of The Blazing Blade for the use of Eliwood and Hector. For the final battle in Rekka, the still-living Athos brought his own tome Forblaze and Saint Elimine's spellbook Aureola. They were sealed away again, until the events of Fire Emblem: The Binding Blade; it is unclear what was their final destiny afterwards, but there are no mentions of a new Ending Winter.