About the arrow: Just one arrow will give your enemies so much badness they will run away with their tail tucked between their legs
This arrow makes use of the new 1.9 tipped arrows with status effects to provide the victim with lots of bad status effects. It has no instant damage effect so you torture your enemy, not kill them outright.
You can put it in your offhand slot so that it gets first priority, but you should use regular arrows when fighting bosses or enderman, or you'll just waste it. (Bosses like the ender dragon aren't effected by status effects, and enderman will just teleport away)
Also, hit a shulker with it when they are open (time it just right), because otherwise it will bounce right back at you.
A direct shot should kill most overworld mobs like zombies, creepers, and the likes.
Arrows with this power can't be obtained in regular survival, so just be wary in multiplayer or you may find yourself kicked from the server pretty quickly.