Handle It

Save this achievement for last unless you get lucky!

The Undead Whelp is the only pet you can obtain from monster drops but is extremely rare, if this is your last achievement prepare to reset the world and boss rush like there's no tomorrow. If you have played Terraria, I'm sorry but I felt the PTSD kicking in too. When it drops it will be very noticeable as it has a bright cyan blue color to it.

My general advice is to have the Cobalt Pickaxe as it makes going down the entire map a roller coaster and an entire setup of items that increase your magic find, preferably on weapons/armor that allow you to take on It with ease. I managed to get up to a 120 magic find along with extra from a crystal I was using that built stacks of gold and magic find buff every time you do damage. This is entirely up to luck and took me 19 world resets killing every mob at each boss floor before I finally got it when I killed It for the 20th time. You may get lucky and find it before your first reset, you may be unlucky like me and have to reset the world numerous times. Just turn on some music, talk to friends, and have a fun time running and slaying everything in your way.