I just started 1 St of my bond quest. What does that do btw
You can gain gems and other small rewards. Mythic bond gains more gems than legendary bonds.
At level 80 you’ll get one or more profile icons. Killerdog made a thread about this bond icons, so you can see which monster icons you like to play for. Missing newer icons are posted by players in a different thread.
After 80 you’ll get more rewards like gems / rare gem, a different font/color monster name and a sparkling maximum bond effect.
The battles get progressively harder, every 10 levels. Up to lvl 80 is usually fine to do, but 80-100 is hard and 100-120 is very hard. You will need a strong setup to do the end! You also need to have the monster +9 to unlock the last 20 levels and you need to build a team with monsters of the same type (4, 6, 8 respectively).
Overall, it’s a place in PvE to spend your tickets to earn gems and other rewards. It’s the end-game PvE content that is practically endless.
This is the thread EBO mentioned: KD's Player Icon previews
Do that buff out monsters?
No, just the visual changes when beating lvl 120 (name goes purple, stars fly up from the base of the monster)