@calebrus44Caster standing next to Mage Slayer casts Misty Step. Is caster gone before MS gets his reaction attack? The caster is gone, since the spell is intended to resolve before the Mage Slayer attack (DMG, 252).
— Jeremy Crawford (@JeremyECrawford) May 30, 2015
What about casting something like Dimension Door?
Epic Melanie
Dimensional door is a little bit more level expensive (4th level conjuration) but has the similar effect than Misty step
And what will happen if caster wants to cast Misty Step near to character with Sentinel feat? Will it provoke OA?
Ilya:
Opportunity attacks happen when the target attempts to leave your melee range by moving away. Misty Step or Dimension Door instantly transports you to the desired location within the spells limits. You never technically “move” out of range, you simply disappear from next to them to another spot.
By that logic, something as simple as Thunderwave renders Mage Slayer completely pointless. If the Mage Slayer is getting pushed away by a 1st level spell before they get a chance to use their reaction, what’s the point in taking it? There are WAY too many spells that will interact in similar ways, making the times the Mage Slayer is actually able to get their reaction attack in the exception, not the rule.