4Game

Give it an active similar to Fae Grenade, where every ~10 seconds you can launch a ranged auto attack.

Currently, Mage Slayer sucks because:

1. You can typically only apply the debuff to one enemy at a time unless you're dusa, ember, etc. (but it's a waste of gold and itemslot on them)
2. Mainly melee heroes use it, so by the time you close the gap, the enemy has already cast spells
3. It's a waste of space late game, so it needs a buff early game to warrant its use
4. The passive is better on pos1 but the stats are better on pos4, so as a result it sucks on both

If we give it an active (think PA's dagger or Fae Grenade), this will address the above 4 issues:

1. You can easily have this debuff up on two enemies (the one you're attacking, and a unit that's further away
2. Melee heroes can use it while they close the distance on enemy casters
3. This will help a bit with farming as it will act as a mini echo sabre (extra attack between auto-attacks)
4. It would actually be viable on pos4 heroes now that it's more utility oriented (decent stats: mana & regen, magic resistance, and ability to reduce enemy spell damage output)