The Host of Turgon was an army assembled by Turgon, the King of Gondolin for the fifth greatest battle against Morgoth, and the only time Gondolin ever participated in any of the great battles of the First Age.
When Turgon had heard of a great mustering against the Dark Lord, he brought forth an army of ten thousand Gondolindrim. Until the main part of the battle started, the host guarded the Pass of Sirion before marching forth at the start of the battle and for the most part, the king was able to restrain most of his people from the rash onslaught started by Gwindor. Then the host of Turgon threw itself into the battle joining the ranks of the western host coming to the aid of Fingon, and together they battled the enemy on the wastes of Anfauglith.
However, the enemy proved too powerful and the host was cast aside and the battle lost. The host survived with help of the Men of the House of Hador led by Húrin and Huor, and they were able to kept the enemy from breaking through the Pass Sirion. Then Turgon, listening to the council of Húrin and Huor was persuaded to return to Gondolin. Battling its way south and covered by the valiant rearguard of the Hador, the host escaped back to Gondolin through the Encircling Mountains at the expense of the Hador who all perished in the Fen of Serech save for Húrin.