How can I improve my pressure based magic system?

Question

My fantasy world Treyidal tentatively has a pressure based magic system. This started as an outgrowth of the inability of my merfolk to do a whole lot, and this is my hopeful solution to construction, forging tools, and combat.

However, I've found it difficult to work on this. For one, I've found it hard to find the information I need on how this would work, and how I could deepen it (like with different forms). I know that on the most basic scale, a stone block is carved of sorts and is used to place pressure on a magical implement called a pestle.

Overtime the energy from the pressure is stored into the pestle, and then by applying pressure later on, such a pestle can be used to heal wounds, other types can be used to harden sand into a stable building material, and so forth.

The problem is is that while I find this interesting, it feels a bit too restrictive for my purposes. Namely, I want the magic to have a combat use, and the magic needs to be able to transform a character into a merperson (obviously with this one they can stamp the pestle all over the human character, but considering I need two characters to be transformed as they are drowning, this isn't nearly fast enough.)

Further explorations in this will involve how this magic is used by humans, and may involve plate tectonics, but I'm not sure how that will work.

Answer 1

1. Block confers magic energy to pestle over time.

2. Various pestles are available for various uses.

ok that is from OP. My add ons

3. It is possible to press a pestle intended for one use into service for another use. Results are less predictable and the skill of the pestle users plays more of a role.

4. Multiple pestles can be used simultaneously to speed the effect. As with anything sped up, this also comes with a loss of fine control.

3 and 4 acting together can be how the merfolk save your humans. They have several pestles available, not all of them correct for the purpose, and they have someone who is good at it but not great. They get it done but there are lasting issues with the end result that play a role in the story going forward.

5. It is possible to make a thing into a pestle that also has a mundane use. This turns up later in the story when someone makes a sword or trident into a pestle.

6. There are pestles that cannot be used because the block was on them too long. These pestles are extremely powerful but have also changed in weird ways as the block pushes them towards the place where the blocks draw their power.

7. It is possible to turn a living being into a pestle. This is towards the climax of the book and a trick your characters invent or propose. One of them becomes a pestle. This is something that the merpeople do not do. There are reasons that they do not.

6 and 7 happened together one time. It is still happening. The thing that used to be a person is still under the block, still being pushed, still growing and changing and accumulating energy. Your characters find her at the end of the book, setting up the sequel.