Darkon's Blight Beast

You infuse necromantic energy into the remains of a tiny or small beast, animating it as your undead pet. 

On each of your turns, you can use a bonus action to mentally command your pet if it is within 60 feet of you.  You decide what actions the creature will take and where it will move during its next turn, or you can issue a general command, such as to guard a particular chamber or corridor. If you issue no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until its task is complete. 

The creature has the statistics of the chosen form with the following changes:

• the creature's type is changed to undead
• the creature gains darkvision out to 60'.
• the creature is immune to poison damage
• the creature is immune to the poisoned condition and exhaustion
• the creature gains vulnerability to fire damage

You may use a 3rd level spell slot to target a beast of medium size.  For every two levels beyond 3rd you may increase the size again to a maximum size of gargantuan at 9th level. You must cast this spell again at least once every 24 hours to maintain control over the blight beast.  When cast in this way you may maintain control over up to 4 beasts of the same size with a single casting.