Does Executioner work on spells?

Executioner is the last ability in the OFFENSE mastery tree. It states:

Increases damage dealt by 6% to targets below 40% Health.

Does this refer to attack damage only, spell damage only, or both?

Solution 1:

Yes Executioner works with spells and more:

The additional 6% to targets below 40% is confirmed to proc on summoner spells, abilities and physical attacks. Does not work with Smite.

Source

Solution 2:

As you'll notice, the OFFENSE mastery page includes buffs to AP and AD stats.

Riot could have made things clearer but the overall answer is yes.

Enemies below 40% HP will receive 6% more magic damage AND normal attack damage.