Multishot currently is hardcoded to use only the default attack projectile of Drow Ranger. This means:

1. The custom attack projectile of her immortal is not used
2. It does not use Frost Arrows projectiles when it is available
3. When used by other heroes (e.g. Rubick, or any hero in AD), it still uses drow's arrow model.

The model should be based on the caster's projectile. Note if this gets changed, be sure to have a fall-back particle effect, for when the ability gets cast by melee heroes in Ability Draft (since melee heroes have no attack projectile set at all).