Bouncing Betty

You quickly put together a collection of assorted odds and ends, imbuing them with some magic (caster's choice: Fire, Cold, or Lightning), and then placing it on the ground.  (you should then move away from the device.) The device becomes active on the start of the next round of combat.  If any creature enters within 10 feet of the device, it is triggered.  When triggered, the device raises 5 feet into the air, and explodes. 

All creatures within a 15 foot radius sphere take:

• 1d8 piercing damage
• 1d6 force damage
• 1d4 thunder damage
• 1d8 (caster's choice from before) damage

All creatures make a dex save against caster's spell save DC.  On success, halve the damages.  On fail, the creature suffers a debuff based on the chosen magic:
Fire: creature is on fire.  At the beginning of subsequent turns, roll 1d4 damage until these flames are put out, either by external magic or an action.
Cold: creature is frozen in place.  At the beginning of next turn, roll a Str save against spell save.  On fail, creature's speed is 0 and they have disadvantage on saves until their next turn.  On success, take 1d6 cold damage, and speed is halved for that turn
Lightning: creature is stunned, and falls prone.  If creature was in the air and cannot maintain flight while stunned, they fall.  Make a Con save against spell save DC to regain mobility on their turn, or they regain mobility after taking damage (other than any falling damage associated with being stunned in the air).

The device is noticeable with a contested perception check against your stealth (If they saw you place it, they roll with advantage, and you roll with disadvantage).  It can be disarmed, if found, with two checks of Dex against Spell Save DC.  A success on the the first allows safe approach, a success on the second disarms the device.  Spell slot is not refunded to caster if device is disarmed.  Creatures may choose to leave the area after a successful first save without attempting disarming.  If attacked while on the ground (AC 5), the explosion happens on the ground, -5 feet to the blast radius.

Ritual casting: Takes 10 minutes to ritual cast, completely submerges the device.  Cannot be identified without DC 20 perception.  Device now has AC 20 for avoiding being hit and the Dex save for disarming is now 20.

At Higher Levels:

• For every Level above 2nd, add 1d8 to the Caster's Choice damage
• For every even Level above 2nd:
  • add 1d8 Piercing
  • add 1d6 Force
  • add 1d4 Thunder
  • add 5 feet of blast radius