Mythic item reasoning:
The majority of Morde players out there build Riftmaker personally I almost NEVER build it because of a few reasons:
The omnivamp is very wasted on Morde while it can have some clutch moments with triumph it really is not worth it as all of mord's abilites are AOE so the healing is being reduced to 33% effectiveness and the original omnivamp on riftmaker is already very low while having decent scaling so it's just very low overall and just not worth it for Morde,
it's also the 9% damage stacking passive really is lackluster as mord's kit is very bursty unless the enemy team has a lot of bruisers/tanks it is just not worth it for how long it needs to stack,
as well as riftmaker got it's AP nerfed to 70 making the difference between it and rocketbelt now 20 AP.
So overall just not worth it and there are much better Mythic Item options for Morde.
Now my preference for Rocketbelt is that it has flat pen on it and the mythic passive is also flat pen which amplifies Morde's Q and overall damage by amazing amounts AND it gives Morde so many more engage opportunites with a dash and MS especially with the rockets applying rylai's slow for even better engages.

Defensive Boots:
Plated steelcaps and Mercs a LOT of Morde players opt for but personally I think sorcs just amplifies Morde's overall damage but especially his isolated Q,
along with possibly being the difference between killing someone in Morde's R and while plated steelcaps can reduce a good amount of damage sorcs just provides way too much value with Morde,
and for Mercs I really only go into hard heavy ap comps or Heavy CC to stack tenacity since it does a lot for him.

My IGN is Magmor where I main Morde, Yone and Cho'gath mid at around 720 LP Challenger rank. Morde was my first champion that I played and loved for 3 years now.
I used to play him top but I find much more success with him Mid.
Ive been playing for almost 3 years now and I have played Morde during that entire time.
I will say all of my guide is my personal preference and through years of experiementing I have determened to be the most viable to 1v9 carry and win games!
I hope this guide helps and good luck in your games!

One of the most common combos I use pretty much every single game is If you land E your Q is garunteed to hit and auto right after which will proc your Passive which can be the end of the trade OR you can all in with Morde's R and run them down if the enemy laner doesn't have some sort of heavy self-peel like say Xerath if u land the combo he literally just dies from 100-0 HP.

But for say something like a Syndra you won't kill her with that because of her Q E combo that launches you away.
You'd need more poke damage for it to kill.
This combo is very easy to use and to get the hang of and it is your bread and butter for setting up yourself to R an enemy in most cases of the game especially when you have Rylai's completed.