Deny the Mage

You have practiced techniques useful in melee combat against spellcasters, gaining the following benefits:

• When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
• When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
• You have advantage on saving throws against spells cast by creatures within 5 feet of you.

Ride the lightning:
Your training has taught you to anticipate the habit mages have of teleporting away from their foes reach.

When a creature within 5 feet of you casts or is targeted by a teleportation like spell or ability, such as misty step, you can use your reaction to do one of the following:

• If the creature is willing, you can chose to be teleported with them, even if the spell doesn't normally allow that creature to take another creature with them. You both suffer 1d6 lightning damage (max 5d6) per level of the spell slot used. This damage can't be reduced in any way.
• If the creature is unwilling, you can attempt to grapple the creature as part of your reaction (If that creature is already grappled by you, you automatically succeed).
  If you succeed, you can choose to either:
  • Be teleported with them, even if the spell doesn't normally allow that creature to take another creature with them. You both suffer 1d6 lightning damage (max 5d6) per level of the spell slot used. This damage can't be reduced in any way. There must be an unoccupied space adjacent to their destination for you to land to do this. If there isn't, the teleportation is prevented as stated below.
  • Prevent them from teleporting. You both suffer 1d6 lightning damage (max 5d6) per level of the spell slot used. This damage can't be reduced in any way (any spell slot or other resource used is still consumed, and other creatures affected by the spell are still teleported).

(Mage slayer  - variant / improved / revised / better)