I have some problem designing a magical spirit with the rules of RoP:Magic, basically I want a spirit with Mentem as his natural form that in his insubstantial form is a dragon with a size of +3.
This spirit will become a familiar and one of the power that I want enchant in the bound is give to him physical form, becoming a real dragon.
In Legends of Hermes is described Monocandus (page 63), a spirit designed using the rules for dragons who strangely can not assume unsubstantial form, moreover it is a magical animal bounded (box page 64) with a ReMe effect.
RoP: M, p. 100 Size is meaningless for spirits, though they may aquire one (along with a body) when using Donning the Corporeal Veil
Cool! Remember to get spirit familiar virtue, mind you.
Also: that'll likely require an impressive labtotal.
Er, no. Monocandus is not a spirit - he's just a magic being (Magical Animal).
The note about him being bound more firmly than other spirits on the island is a tad misleading in this.
You prospective familiar is natively a spirit that just happens to take the shape of a dragon, as far as I can tell.
If so, use the rules for spirits, and notice how Donning the Corporeal Veil increase in price for a body larger than size +1.
You could think that is a spirit that can't be airy anymore because one mystic condition: one pact or something, any faery is suposed be one type of spirit that can coagulate just like the demons and angels; the magic beings are first their Essential Nature, so they are spirits and phisycal form one time too, against the humans that are composed of soul, mind and body.
You can consider that a dragon can have some attributes of spirits/daemons if you want it.