If Boros Reckoner is dealt damage by multiple sources at once, such as by two creatures blocking it, its ability triggers once and one target is dealt that much damage.
14-03-2017
wotc
Boros Reckoner's first ability will trigger even if it is dealt lethal damage. For example, if it blocks a 7/7 creature, its ability will trigger and Boros Reckoner will deal 7 damage to the target.
14-03-2017
wotc
Damage dealt by Boros Reckoner due to its first ability isn't combat damage, even if it was combat damage that caused that ability to trigger.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.