Relentless Spear

The Relentless Spear is a treasure in. It is actually a dart with a blue flight and shaft, as well as two blue lines on its barrel.

Below-Grade Discotheque
There is a Relentless Spear inside of a Tusked Blowhog near the start of sublevel 4 of the Below-Grade Discotheque. To collect this treasure, you must break the dirt wall, then defeat the Tusked Blowhog, and lastly throw 7 Pikmin at the treasure.

Hero's Hideaway
There is a Relentless Spear stuck to a corkboard on the western edge of the overworld, pinning down the Wisdom Emblem. To collect this treasure, you must first knock it down with a rush from Oatchi, then throw 7 Pikmin at it.

Ultimate Testing Range
There is a Relentless Spear lying on a hovering platform at the southwestern corner of sublevel 1 of the Ultimate Testing Range. To collect this treasure, you must stand on the other moving platform, then throw 7 Pikmin at it when you're at the right height. You will need to construct the clay bridge to collect it.