Is it possible for the mob to grapple with the player

[ivory spoke]

There is no vanilla behavior for that, so you will need to implement that yourself.

First split it up into multiple small questions that can be solved individually.

• What will the mob and player be doing to trigger the grappling state?
• How can I prevent the player and mob from moving?
• How long should the grappling state last before the player dies?

For question #1, you might look into using a behavior like the charge attack, which makes the mob move quickly towards the player like a vex does. You can also query for when the mob is charging, so then it is a matter of testing if the player gets close enough to the charging mob to initiate a grapple.

Grappling would really just be preventing the player from acting, so we have a few options for that. You could give the player Slownes to prevent them from moving; you could have the player teleport in front of the mob constantly to prevent them from moving, or summon a rideable entity that they are forced onto. There is no wrong way to do it.