The dragon Stellatus on page 194 has an intensity of +15 in the description of his Fiery Breath. I've looked and I can't figure out what intensity is, when to use it and how it is used in game terms. There is no mention of it in the index. Help?
No idea. It looks like the description is still fully functional if you ignore this statement completely.
Isn't it additionnal penetration, or something like this?
I think this refers to the Heat and Corrosion Table in the Obstacles Chapter. The intensity of Stellatus's Fiery Breath is given as +15, which is hotter than molten iron, so that complete immersion yields +60 damage (x4), half immersion +45 (x3), a limb +30 (x2) and only a small part affected is the base damage, +15. These numbers don't quite match the power description, though, and I'm not sure how you would design a spell to use them. My guess is that intensity doesn't matter when the spell or power creates fire (though note that the MuIg, PeIg, and ReIg guidelines all mention intensity increasing the difficulty of appropriate spells), and that Stellatus's breath weapon is roughly equivalent to a Level 45 Hermetic effect (R: Touch, D: Mom, T: Ind). Maybe this is provided in order to help the storyguide decide if things other than people and animals catch fire or melt when exposed to the flames?