If you have completed the last 3 Collector tasks, you will unlock the quest: In search of power!

This is a special mission that, when completed, will give you as a reward:

1x Shadow Harpy card hench (Lv.175)
+1.000.000 EXP

To complete the mission, take the last henchs in research at the maximum level and some items to the collector in Mekrita. 

PsychoWags at level 155
You can mix using: 
Wagstuff (2nd mix item) + Mr.Rupert (2nd mix item)

SnowRush at level 132
You can mix using: 
RockyRush (2nd mix item) + Mint Lion (2nd mix item)

Special Crab King given at last quest, at level 150

100x Rose
Can be obtained by hunting Force Flyer (at Islaba Damp Area)

30x Seaman's Essence
Can be obtained by hunting at Pirate Island

10x Seed of Purification
Can be obtained in Elysion NPCs.

You need to have completed all 3 tasks from Collector before to be able to receive this quest.