If you attack with
Dirge and it does more than enough attack damage to KO the defender, Steamroll’s ability will work, but Dirge’s ability won’t trigger. If Dirge does exactly enough damage to KO the defender, Steamroll won’t do anything.
Sure. Let’s say your opponent has a character with 12 health and 9 damage counters on it. You attack with the chosen character and would do 6 damage to it. That’s 3 more than you need to KO the defender, so your opponent chooses one of their other characters and puts 3 damage counters on it.
What if your opponent puts those 3 damage counters on a character with 10 health and 9 damage counters already on it? Do the extra damage counters on go to a third character?
[Source: WotC] 
No. The “Steamroll damage” only carries over to one other character. That 10-health character would end up in the KO area with 10 damage counters on it. The extra 2 damage counters are lost.
Does the damage counters my opponent puts on another character count as attack damage?
[Source: WotC]
Yes. The damage counters represent attack damage done by the chosen character.
Say Dirge (In bot mode) gets the effect of Steamroll, and then KOs a character, but does too much damage and ends up with 3 damage left over. Following Steamroll's effect, that 3 damage gets moved to a different character, but the damage spillover target has only 3 health left, and the second character is KO'd. The first KO obviously doesn't trigger Dirge's bot mode skill, but would that second KO cause Dirge's skill to kick in untap him?
[Source: WotC]
When you move the damage via Steamroll, it ceases to be attack damage and therefore doesn’t trigger Dirge’s bot mode ability.
What occurs if I attack with
Volcanicus the turn I combined while also playing Steamroll that same turn (assume I used a
Brainstorm or Multi-Mission Tool on Divebomb) and flip enough Orange pips to KO the defender with Volcanicus' ability. Do I still calculate my total ATK damage to determine what (or if) Steamroll's damage would be?
[Source: WotC]
In this case,
Volcanicus has KO’d the defender before actually doing attack damage to it, so the attack will end with no attack damage dealt. Thus, Steamroll’s effect would not apply
No. Both abilities are practically identical, so one of them will apply (you can choose), but then the other one will not apply.
My group has found conflicting answers in regard to Steamroll’s damage carryover being considered "combat damage" or not. FAQ on the app states that it is Attack Damage. Yet, Roundup 2/28/19 states that
Motormaster would protect against Steamroll (making it non-attack damage) and Roundup 2/22/19 specifically states that Steamroll is NOT attack damage.
[Source: WotC]
It’s possible you might be looking at an older iteration of the FAQ. There were conflicting answers earlier in the set’s life, but Steamroll’s damage carryover is not considered attack damage.
Would playing multiple instances of Steamroll in the same turn allow for damage, if it were high enough to KO the second character as well as the initial character that was knocked out, to carry over to a second or third target? For example:
Brainstorm into two Steamrolls or play Steamroll and then flip
Bumblebee - Trusted Lieutenant into alt-mode and play a second or even a third Steamroll (if Brainstorm was played first).
[Source: WotC]
Playing more than one instance of Steamroll on the same character has no additional effect. The extra damage will carry over only once.
If I have 3 characters and one of them is
Motormaster in alt mode. My opponent plays a Steamroll and does more than enough damage to KO one of my other two characters. Do I have to put the extra damage on Motormaster or can I put it on my third character? If I put it on my third character does Motormaster's ability cause that damage to be lost or does the Steamroll effect supersede Motormaster's ability?
[Source: WotC]
You can choose the third character and
Motormaster will protect it. No extra damage is done.
What happens when you put the Steamroll action on
Grimlock - Dinobot Leader and he KO's a character with excessive damage? Does it only do the effect of one or the other, or does it stack?
[Source: WotC]
Only one effect will work. There’s no extra damage after the first effect applies.
You can play the Steamroll, however you cannot use it for
Nightbird's attack as by the time Steamroll is played, Nightbird's damage was already applied and the defender was already KO'd. If you happen to attack again on that same turn, Steamroll could be usefully applied to an attacker.
How does Steamroll and Grimlock, Dinobot Leader's bot mode ability work when KO'ing an undamaged character with
Safeguard?
[Source: Aequitas]
Attempting to Steamroll into an undamaged character with three health, such as Slammer, Combat Drone, and
Safeguard 3 (for example
Medic's Protective Field) will first see the damage reduced to 3 damage, and then KO the defender. No additional attack damage would be available to be steamrolled into another enemy.
No - the Autobot was not in fact KO'd.
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