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?
Yes Executioner works with spells and more:
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.