Astral Grasp

You've gained a feint presence beyond material plane, and on rare occasions can momentarily glimpse and interact with the immaterial.
When a creature attempts to teleport, change dimension, move while in a non-solid form, or change into a form that isn't their native form, and the attempt either starts within your melee range or its path passes through it, you can use your reaction to reach and grasp them, forcing a contest between your Strength (Athletics) and appropriate Ability Score/Skill that the target is using for their attempt. (Usually spell casting, or highest Ability Scores + Proficiency Bonus for innate abilities.) You must have line of sight to either the starting position of the attempt or its destination, and have a free hand or be already grappling the target, in which case they have disadvantage in this contest.
Should you win the contest, you can choose to deal force damage to the target equal to one unarmed strike, and affect them by one of the following:

• Subject the target to the effects of being Shoved, either knocking them prone or push them in any direction you choose. If they're teleporting, this takes place after their teleportation is complete, and you can decide based on their destination.
• Reduce the movement speed of the target to half for 1 round.
• If the target is teleporting, move the destination up to 20 feet towards you. The target can choose to teleport to the new destination's closest unoccupied space, closest unoccupied space witch a surface which can support them, or remain in their current place in which case only Action/Bonus Action/Reaction/Movement used for the attempt and no other resource (e.g. Spell Slot) is lost.
• In case you roll a critical success or the target rolls a critical failure, you completely pull away their immaterial essence back to reality. The target reverts back to their natural form and if they're teleporting, it is intercepted which causes the target to remain in their initial place if they started within your melee range, or if they didn't, materialize in a place of your choosing within melee range that intersects with the path of teleportation. You can choose to deal from both types of force damage and bludgeoning damage, effectively doubling it. Also you can choose to have the target stunned until the end of the turn, grappled, prone or all three. Finally any Action/Bonus Action/Reaction/Movement used for the attempt is lost, and other resources such as Spell Slot are lost only if the attempt was partially complete. (e.g. a teleportation that was intercepted mid-path.)