Relentless Arduran is a unique monster. It starts at level 1 harmless Arduran. You have to feed it for it to grow to the level 99 unique monster. Before that don't kill it!
The best 40–60 feed item is ../BloodWalnut, which is +2. The +5 item ../NamekoGem is super rare.
The best 60–80 feed item is ../MelodiousMelon.
The best 80–100 feed items are particularly rare. The best one is ../BellflowerCrystal.