User blog comment:B214/Question and Answers 6/@comment-180.253.126.123-20160717091644/@comment-24.113.252.213-20160717162708

I'm not so sure it's a "True Form", as in what we see him usually look like is just a transformation or illusion. I believe Sirzech's "Human-Shaped Power of Destruction Form" is actually something like this:

It was mentioned that Sirzechs focused all his magical talents into mastering his Power of Destruction, and the primary reason he was chosen as the new Satan Lucifer was because of his mastery of it.

Imagine in Dragon Ball Z (Ishibumi is a fan of this series, and likely took some Ideas from it). The main characters keep their power level low, but are still able to fight and do basic stuff with their Ki. But when the situation calls for it, they can "power up" to a higher level of strength (I'm not talking about Kaioken or Super-Saiyan), and then do what they were doing before, but faster and stronger.

I think Sirzechs does the same thing. We know that his strength is Satan-Class when he is "powered down", and we've seen him fight Issei in Vol.8 while still "powered down." Then he "powers up" to his full strength, as we see in Vol. 12, by amplifying his Demonic Power/Power of Destruction, and concentrating it with his magic talent. It's basically the same principle he uses to make his "Ruin the Extinct" balls he used against Cruesery Asmodius, and Issei.