10% of level + 10
Magic Dart (also referred to as Slayer Dart) requires 50 Magic and 55 Slayer to cast; in addition, a slayer's staff or Staff of the dead must be equipped to cast it. Slayer dart is popular among magic users when doing the Barrows minigame as it hits frequently, and the runes required to cast the spell are a common reward from the minigame.
The maximum hit of slayer dart is equivalent to 10% of your magic level, plus 10. So if your magic level is 60, the max hit would be 16. If your magic level is 70, the max hit is 17, and so on. The spell hits a maximum of 19 at 90+ Magic.
This spell's accuracy is based on your magic attack bonus. The common misconception that it isn't is due to the fact that many players use it in the Barrows minigame with a full set of armor. Barrows brothers have no magic defence at all and can be hit with any spell while in full armour; there is nothing unique about this spell hitting them with low magic attack.