Just a few notes about this build.

It's more of a carry build than anything but you can still absorb quite a bit of damage as most decent Mords do, giving you some tankability.

Opening with regrowth pendant is a personal preference. If you want a shield you can go that rout but it slows down the build slightly. Getting heart of gold in general seems frowned upon however I like getting it like this because it sits in your inventory paying for itself and then stockpiling more gold for quite some time before you build Randuin's. Sorc boots make you hurt all the more while still giving you mobility. If you are facing a heavy stun/diable team feel free to switch to Merc treads. Getting Rylai's next gives you both the ability to kill effectively and survive for some time. The movement speed provided by FoN makes catching runners a joke with Rylai's as well as giving you quite a bit of regen to stay in the fight. The obviousness of getting Sunfire and then finishing Randuin's should be obvious (armor+regen+damage+survivability) making you even harder to kill and even more annoying at all times. Lastly we build Spirit Visage just to put our regen over to the top and add a bit more magic resist.

By the time you get to Sunfire you should be able to take on 2-3 enemy champions and win. By the time you finish the full build you should be unkillable unless you're out playing by yourself and you get jumped by the whole enemy team and have no support from you team, which you should be doing anyway.

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Runes should be obvious, you can swap the glyphs for CdR, however by the time you get to the end of this build and in the event you still want more CdR grab the blue and laugh at how fast you can spam your abilities.

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Skill Sequence should also be obvious, SoD does good damage by the time you get your Rylai's and only gets even better. You can get a point in MoS earlier if you want trading it for one of the points in Creeping Death.

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Lastly summoner spells
Ghost is awesome for escaping and general map movement if you need to get somewhere quickly, and ignite is great in combination with your ult. I would not use anything but those two with Mord but there are other options. As long as you aren't using revive.

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Thanks for reading, I hope you enjoy Mord as much as I do and hopefully you'll play better than I do.