Volumen Hydrargyrum

You create a Large chromatic blob of quicksilver in an unoccupied area within range. It is an object given form purely as an extension of your will and has no stats. If it is targeted by an attack, it is considered to have AC 10, but the attack goes through it and has no effect. It cannot be destroyed by any means except by being dispelled. The blob lasts until your concentration ends, until dispelled, or until you are separated from its closest point by more than 30ft. You can end the spell as a free action, at which point the quicksilver re-enters its vessel. If it is dispelled, the blob loses all form and immediately spills as it reverts to regular quicksilver.
The blob follows you without instruction to the best of its ability. It can climb up walls and ceilings, slip through holes as small as 1 inch in diameter, and extend itself up to 30ft at a time in any direction. When you cast the spell, and on each subsequent turn, you can use your bonus action to manipulate the blob for one of the following abilities:

Scalp. You make the blob shoot 2 hydraulic tentacles to whip at targets within 30 feet of it that you can see. Make a ranged weapon attack centered on the blob for each whip. The attack uses your spellcasting modifier. A whip deals 4d8 magical bludgeoning damage on a hit. You may choose the same or different targets for each whip.

Shield. The blob spreads into a thin, semisolid form to protect you from any attacks. Until the start of your next turn, any damage roll targeting you that deals less than 10 hit points of damage is completely blocked. Rolls that exceed this have their damage reduced by 10 as they splash through the barrier. Psychic damage is not affected by this ability.

Probe. The blob darts out countless tiny tentacles searching every nook and cranny for movement. Until the start of your next turn, you can detect any living creatures or moving objects within 60ft of the blob so long as there is a nondestructive path to them from the blob.

Harden. You channel magic throughout the quicksilver to force it into a rigid shape until the start of your next turn. It can be composed of a contiguous set of up to 10 5x5' panels of 1" thickness. At least one panel must have contact with the floor or a structure. While in this form, each panel has AC 20 and 30 hit points, and a panel can provide total cover. If the panels are no longer contiguous, or no panel is supported by the floor or a structure, then the blob immediately reverts back to liquid shape and congeals near you, and you temporarily lose this ability next turn. If the blob is already hardened at the start of your turn, you can rearrange its structure as long as it's connected to the floor or a wall at all times.