The lance was safeguarded for years but one of the latest House Gautier children, Miklan, stole the Relic after he was disowned by his family when he did not inherit the Crest of Gautier while his younger brother, Sylvain did. His theft of the lance lead to their father, Margrave Gautier, to request the Officers Academy to dispatch its students to reclaim the lance. Miklan uses the lance to battle the Officers Academy students, even possibly facing Sylvain himself. The students easily defeat him and in desperation, he attempts to unleash the Lance's power, only to be overtaken by malevolent energy. Due to not having a Crest, the lance transforms him into a terrifying beast, forcing the students to defeat the creature. After a hard fought battle, the beast is slain, allowing the students to recover the lance from Miklan's corpse and return it to House Gautier.
Later, Sylvain receives a letter from his father to quell Miklan's remaining bandit gang who are ransacking a local village. Though he requests Sylvain to complete it alone, he asks Byleth and his fellow students to help stop the raid. After all of the bandits are dispatched, Margrave gives Sylvain the Lance of Ruin as he is the rightful wielder of it as the next heir of House Gautier.
If Sylvain is in your chosen house during "The Gautier Inheritance" mission, you are able to decline Lady Rhea's request to give the Lance of Ruin to the church immediately after the mission. While this will anger Rhea slightly, it will allow the player early acquisition of the Lance of Ruin.
Lance of Ruin
Hero's Relic (Crest of Gautier)