What was Incursio's trump card ability? 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?
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.