Quintessence (called Aegir in the Japanese versions) is said to be the element that dwells inside every person, as his or her own life force. When living things die, their quintessence is released and the power can be harnessed by skilled magic users.
It is mentioned briefly that the Valkyrie Staff could only revive people with substantial quintessence.
In Fire Emblem: Rekka no Ken, Lord Nergal, a Dark Druid, discovered Arcadia with his friend Archsage Athos, one of the Eight Heroes. Using the archives of the Dragons, Nergal discovered how to harness the power of quintessence to create life, greatly extend his lifespan and heal otherwise fatal wounds. Using morphs, or emotionless human clones born of heresy, Nergal launched a campaign against Lycia in order to intercept the quintessence created by the war and become even more powerful. He desired to use this power in order to bring dragons back into the land of Elibe through the Dragon's Gate on Valor in an effort to get the Dragons' extremely strong quintessence and become all-powerful. However, Nergal was defeated by the efforts of Eliwood, Hector and Lyn, nobles from Lycia.
In real life, Quintessence, was thought to be the fifth of the elements, thought to make up the chemistry of existence in medieval times. The four others were considered to be fire, water, earth, and wind. "Quint," meaning five, and "essence" are combined to make what is essentially just a more romantic and sophisticated word.