What was Incursio's trump card ability?

Question

According to the definition of a trump card, it is the ultimate Teigu's ability, one that often comes with a risk to the user's life.

It is true for several Teigu, like Murasame (infecting oneself with the fatal poison and the regret of everyone slain by it), Susano-o (giving up one's life force to the Teigu, said to be usable only three times before certain death).

Wikia has on the Trump Card:

Several Teigu were shown to have a hidden ability (奥の手, oku no te, lit. secret move), often referred to as "Trump Card" or "Ace in the Hole". (...) Usually, such moves are the most powerful, last resort ones. _{(emphasis mine)}.

Wikia lists the invisibility power as Incursio trump card. It goes against the very definition, since it has no risk (exaustive, yes, but not life-threatening) and is used in almost the time.

So I have to disagree with that definition. I think that Incursio Trump card is the evolutionary power. It is shown in the Manga that Tatsumi is being consumed by Incursio due to the repeated evolutions:

However because Tatsumi forced Incursio to evolve rapidly to increase his strength, Incursio started to manifest in one of Tatsumi's eyes. After being examined by a doctor, he reveals that the armor has started to bond with him. He also states that after a few years Tatsumi could use it without overdoing it like Bulat. Now at the risk of being devoured by Incursio (...) _{(emphasis mine)}.

So, what is the trump card of Incursio? Is there any official or canon material on the issue?

Answer 1

I do not think that the adaption and evolution can be counted as a trump card.

Before Tatsumi, Bullat and Gensei also were users of the Teigu Incursio. Both Bullat and Gensei did not unlock the capability of evolution in the Teigu like Tatsumi did. This might be due to them not being skilled enough to or just because Tatsumi has so much hidden potential.

With that said for most Incursio users the invisibility most likely will be the actual trump card, with the only life losing risk being

As noted, a weakness of the invisibility is that while it masks the user visually from the naked eye, it does not hide their "presence", allowing skilled opponents to sense the user despite the invisibility.

The fact that Tatsumi did manage to awaken this power from the Teigu, most likely is due to his hidden potentional, which seems to get stated by allot of characters through out the series as well

Tatsumi's name means "Sleeping Dragon", which seems to be a reference to his hidden potential, which has been noted by many different characters in the series. The first creature he was shown fighting was an Earth Dragon, and he later obtained the "Demon Dragon" Teigu.

Answer 2

I think the fact that it does not render true invisibility to the user is a great weakness. In one episode, Chelsea finds tatsumi's presence in the hot springs. The fact that you enter a battle with an ability which is fallible is as great a weakness as Murasame. Because when I watched the anime, the fallible nature of invisibility seemed too much of a weakness to me to ever use the ultimate technique. This seems like an abstract reasoning. So let me explain it in comparision to murasame.

Murasame infects the person who is cut with a fatal poison and knows neither friend nor foe. The fact that akame can use it depends on her skill to handle the sword. Similarly , Incursio's invisibilty also depends on the user's abilty and strength and skill. One misstep can kill you in either case. I would believe this was the thought process in creating the weakness of incursio. Also each teigu has it's own set of weakness and strengths. You cannot expect each one to kill the user. For example the Omnipotent Five Sights teigu would make the user hear voices in the head which not necessarily kill the user. Or Mastema which has no such weakness(afaik,correct me if I am wrong). So saying incursio's trump card ability is not limited is wrong. I think we are too interested in the protagonist having the strongest ultimate and a corresponding weakness to believe that tatsumi can just make himself invisble.(To relate this to another anime, kenpachi zaraki of bleach has no bankai and is still hell of a lot stronger, this is akin to that in the sense , incursio's power lies in its ability to adapt and evolve and have almost limitless strength rather than its ultimate ability). Hope this helps :D