If you're feeling brave/skillful, you can go for an AD build. 9/0/21 masteries(minus any mana regen ones) CDR per level glyphs/quints Magic pen marks and whatever seals(Health regen/flat CDR/Health are all good ones.)
The build is simple, first, start with a pure health regen item, which will be sold later and one potion. next, build boots of swiftness and phlange, this is to compensate for morde's lack of a closer so always try to approach from somewhere least expected, it will require some skill as a ganker. next, build abunch of BF swords, four to be presice, once you have three, sell off your health regen item to make room for the next, then, start turning them into blood thirsters. if you feel like you're being taken down too easily, invest in the fullblown icehammer.
There are a few important things to note however. For starters, this build doesn't really change, so it may be pretty static for most people. If you have a spell resist tank, switch out the boots for sorc shoes and one of the swords for...I'm just kidding, don't ever do that, just do as normal, it won't matter too much because alot of your damage will still be in the form of auto attacks(Ideally with 100% lifesteal)
So I suppose you COULD switch out one of the swords for a void staff if you gotta be lame like that :P but really, I would honestly just stick with the swords, for a few reasons, to start, no matter who you hit in a group fight, when you're spamming those mace of spades and possibly creeping death(There's a reason for all that CDR) everyone will be taking damage from you no matter what, the only difference in who you're targetting is that they're taking all of your auto attacks ontop of that so you can still damage the tank while beating down the carries and then finish off the weakened tank when the carries are dead, second (And more relevant), magic pen and AD do not go hand in hand in terms of itemization, and honestly, you're almost gimping the build by stacking any more than what your boots and masteries/runes provide, I of course have only tried this use of additional spell pen once, so feel free to experiment if you even dare to try it to begin with
(PS it will probably be really hard if you wheren't already really good with morde)