To start the 21. The Gladius side quest in Trinity Trigger, speak with Glaive after completing the 20. The Axe side quest. For this quest, you’ll need to find an altar in the Gladius, which is the dungeon you’ve already beaten near Woodroost.
Once in the Gladius, use the teleporter to reach Gladius F3. On the western side of this area, a new way will now be opened revealing an unexplored area! Once you make it to the altar, report back to Gladius to complete the quest. As a reward, you’l receive Sword for Elise and Zantis, EXP Booster IV and Evasion Recipe II.