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?