Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Magic Market Index for March 9th 2018
Trample (This creature can deal excess combat damage to defending player or planeswalker while attacking.)
If another red source you control would deal damage to a permanent or player, it deals that much damage plus 1 to that permanent or player instead.
6/22/2015 Embermaw Hellion’s last ability doesn’t cause Embermaw Hellion to deal damage; it affects the amount of damage dealt by the original red source.
6/22/2015 If the player or permanent being dealt damage is also affected by a damage prevention effect, that player or the controller of that permanent can apply that effect and Embermaw Hellion’s effect in any order. If all of the damage is prevented, Embermaw Hellion’s effect can’t apply to it.
6/22/2015 Multiple Embermaw Hellions are cumulative. If you control two of them, you’ll add 2 to the damage dealt by another red source you control. In this case, you would also add 1 to the damage dealt by either Embermaw Hellion.