From MFL.com:
How do I score the 2 Point Conversion Return by the Kansas City Chiefs in Week 13?
Answer: In the Chiefs/Falcons game, the Chiefs scored 2 points as follows: The Falcons were attempting a 2-point conversion, and Matt Ryan threw an interception as part of the attempt. Eric Berry intercepted the pass and and returned it all the way back to the opposite end zone. Since this happened on a 2 point conversion attempt, it is scored as 2 points for the Chiefs. It is a new rule the NFL put in place starting in 2015. It is a very rare occurrence, but you can add this scoring rule to your fantasy league to cover this if desired.
Note that the stats on a 2-point conversion are not counted towards a player's stats for the game (and you won't see them in any NFL box score either). The play is just considered a "successful" or "failed" 2 point conversion, so any yardage or receptions or interceptions that occur as part of a 2 point conversion attempt are not included in the regular stats for the game.
The scoring rule is called "Defensive Conversion Returns". You can add it to your league on the Reports > Rules >
League Scoring screen. To make a change, click on the (Edit) link next to the "Team Defense" Position group to add the new rule, and assign it a fantasy point value.
Note: This is a new scoring rule that the NFL added in 2015. It happened only once last year in Week #13 of the 2015 season, when the Saints blocked an Extra Point attempt and returned it all the way for 2 points. For 2016, this is the third time it has happened. The first time it happened was in Week #2 with the Ravens against the Browns, and then it happened again in Week #10 in the Broncos/Saints game. In all of those previous instances, it happened on a blocked extra point attempt.