On the other hand, there is strong circumstantial evidence that Diedne WAS a Mystery Cult House rather than a True Lineage or a Societas.
I'm in a hurry so please excuse me if I'm just going to re-post my long-standing guidelines for Spont specialists, they ought to be appropriate for Diedne in most cases.
If your House and background is compatible (no closet Diedne Tremere, sorry), by all means do start with Diedne Magic. It is simply unvaluable to a serious spont specialist.
Enduring Constitution and Spell Improvisation are also good complementary Virtues to have. The former more so than the latter.
Life-Linked Spont Magic is but the poor substitute for Diedne Magic if the latter isn't possible. A Diedne magus initiating it, however, will make for a very powerful combination.
Same deal for Faerie-Raised Magic, to a rather lesser degree.
Chthonic Magic may be a rather better (if socially questionable) substitute, and combining it with Diedne Magic (by initation, or if your GM will let you, playing an Ex Misc with Diedne Magic and Chthonic Magic as the extra Supernatural Virtue, e.g. a Thessaly Witch offshoot) will make for a quite powerful and flavorful (curses and sacrifices !) combination.
A spont specialist magus should in many cases naturally be interested in mysteries that may significantly enhance spont magic: the best ones are the Merinita illusion and charm schools (Faerie Magic, Charm Magic, Story Magic, Animae Magic, Spell Timing and Glamour magic are all very good Virtues for a spont specialist, esp. Charm Magic) and theurgic Name magic (Hermetic Theurgy Form spell spirits make for a rather handy and flavorful substitute for non-Diedne spont magic specialists, while Invocation Magic may allow very siginificant boosts to spont spells; Synthemata Magia may be quite good to have if you often deal with supernatural creatures from any realm). Consummate Talisman and Inscription over the Soul may also be remarkably useful (casting bonuses from Charm Magic, Invocation Magic, and a Consummate Talisman that is incorporated in your body/spirit all stack).
As a general rule, any Virtue that opens up new R/D/T or spell guidelines to non-ritual magic is a godsend to a spont specialist, except Holy Magic and Hermetic Geometry for thematic and rule issues.
Harnessed Magic may be useful to discard sponted spells (typically Corpus, Imagonem, and Mentem) that are no longer useful.
If you often casts spont spells subtly, esp. in the Dominion, or while shapeshifted, getting the Subtle/Quiet Magic combo may be quite advisable.
Oh yeah, I also forgot to mention that a spont specialist should do very well to maximize Arts (cases may be done both for a generalist approach or for specializing in 2-3 Techniques and 3-4 Forms, but owning or initiating Skilled Parens and Virtues that speed up Art learning, such as Book Learner, Free Study or Study Bonus may be rather useful; the latter two combo makes for a flavorful thematic combo with the spont specialization), and Arcane Abilities (Artes Liberales, Concentration, Finesse, Magical Theory -for spellcasting bonuses cap if nothing else-, Parma Magica, Penetration, and all the Realm Lores) as they are your dear and valued adventuring friends; pay them the attention and cares they deserve. Remember to develop your Mysteries' Lores to advance.
I am very fond of Unaging and (Strong) Faerie Blood. Plenty of added time to master your mysteries, and standing free invitation into Merinita.
Sorcerous Magic may be a good substitute for Quiet/Subtle Magic to do the sneaky subtle bard/musician-mage in the Dominion.
You might also pick some Flaws that round up the specialization: I suggest Careless Sorcerer, Cabal Legacy, Covenant Upbringing, Loose Magic, Magic Addiction, Pagan, Restriction, either Rigid Magic or Unstructured Caster, Unpredictable Magic, Weak Enchanter, Blatant Gift, Depraved, Fury, Personality traits and Flaws that indicate a problem with impulse control (Ambitious, Proud, Lusty, Wrathful, Optimistic, Carefree, Overconfident, Reckless), Fettered Magic, Poor Memory, Poor Formulaic Magic, Prohibition, Social Handicap, Supernatural Nuisance, Vow, Vulnerable Magic, Warped Magic, Weak Magic Resistance, Weird Magic, Greater or Lesser malediction, Lycanthrope, Sheltered Upbringing, Faerie Upbringing, Chaotic Magic, and Visions. All of them make for either good Flaws to take at creation or initation Ordeals.
For the life of your character never ever pick Enfeebled, Harmless Magic, Unnatural Magic, Holy Magic, Mercurian Magic, Clumsy Magic, Painful Magic, Difficult Longevity Ritual, Short-Ranged Magic, Study Requirement, Weak Spontaneous Magic, Difficult Spontaneous Magic, Deficient Techniques and Forms, Disorientating Magic, Short-Lived Magic, Slow Magic, Weak Magic, Weak Parens. They will all interfere with your chosen spont magic focus to some degree.