The Stag card says that if you do at least one damage then you do a free damage. The Mithril Armor card prevents one damage. So do you get the free damage and still do one damage, or does the Mithril prevent the first damage so no bonus is given?
Great question. Damage prevention happens before things that trigger off of damage, so Aspect of Stag's extra point won't trigger if Mithril (or the Umenzi army ability) prevent the only point of damage dealt by the attack. Similarly, if a Lizardmen unit with Fury deals two points of damage and Mithril prevents one, the Lizardmen player cannot erase the mark to do an extra point.