Channel Agathys

You transform your flesh into living ice, the coldness of which can be felt a number of feet away from you. For the duration of the spell, or until you dismiss the spell using a bonus action, you gain the following effects:

• you can make a melee spell attack against dealing 3d8 cold damage if the attack hits.
• If a creature hits you with a melee attack, the creature takes 1d8 cold damage. 
• You gain resistance to cold damage but vulnerability to fire and force damage.
• By touching a body of water, you can use an action to freeze 30 cubed feet of water. Each 5-foot-square frozen section has AC 5 and 15 hit points. Reducing a frozen section to 0 hit points destroys it. 

At higher levels, when you cast this spell with a spell slot above 3rd, the damage dealt by this spells melee spell attacks increases by 1d8 and the dealt when a creature hits you with a melee attack increases by 1d8 for each spell slot above 3rd.