Nightmare cannot be removed by basic dispels like Eul's or Lotus, so it is a reliable long disable.

Brain Sap always deals full damage ignoring magic resistance, so at level 5 with two points you can solo pressure offlaners.

You can Nightmare an ally right before a Laguna Blade lands, the 1 second invulnerability makes the damage fizzle.

Nightmare does not pierce spell immunity, so remember the enemy carry pops BKB.

Aghanim's Shard makes Brain Sap apply a 1.5 second Nightmare, letting you chain Brain Sap into Nightmare into Fiend's Grip.

Bane controls fight tempo by removing the most important enemy hero, so save Fiend's Grip for the priority target.