This build is heavily focused on being a menace in team fights. Your E is one of the strongest abilities in the game and can easily turn the tide of battle if used correctly, so we are going to buff up our ability to use it as effectively as possible by being tanky enough to walk up to get that perfect E, being fast enough to actually catch people in it and have tons of CDR to spam it as much as possible.

Everfrost is simply the best mythic for veigar. I have tried others such as locket which is definitely good but leaves you lacking damage which in turn makes it harder to stack and get more damage, also you don't have any mana from locket which pretty much forces you to go frozen heart second when we really want dead mans as soon as possible.
Dead mans is essential in the build, we need this to be able to catch people, escape danger and roam quickly.
Vigilant wardstone is not essential but too good to pass up on, I don't think any other champion build gets as much use of of this item as this build does. Its super cheap, gives us even more vision and gives us a percentage increase to all the stats we want. Insane item. can build the MR component from abysall first if you really need to.
abyssal mask isnt exactly essential to the build and the game will probably end before you get it but we need an MR item and this one just makes the most sense and is the cheapest.

Guardian makes our early game better which is our weakest point in the game
font of life isn't amazing as it can be inconsistent but its better than the other options and since we build a lot of health we get good use out of it.
bone plating for even better early game, simply the best in slot.
unflinching is great because we already get a lot of health so overgrowth is simply overkill, the tenacity will help us stay alive a lot more than 150 odd health ever would and we get the benefits immediately rather than having to wait to scale.
manaflow band and transcendence are essential mage runes and dont need an explanation.

As soon as you join the game tell the adc that you will stack with Q when relic is up and you will ping when to avoid missing stacks. try poke the enemy with q whenever you can without putting yourself in danger. The early levels are your weakest but you still have kill pressure with your E however don't waste it as it is also your disengage and is needed for escaping ganks.

your biggest power spike is when you have everfrost, cdr boots and the MS component of dead mans, when you have this you have every thing you need to be a strong teamfighter and everything after this is just extra. once you have deadmans that is also a massive power spike as you should be tanky enough to bait enemies into trying to one shot you when they wont have the damage too.

its simple, land E in teamfight. There are many uses for your E whether it be zoning, stopping escapes, peel or trying to burn summoners. everything else if pretty simple just hit them with Q and W and ult priority targets once they are around or below 50% HP

pros

• very tanky for a mage
• scales insanely hard
• strong team fighting
• not as weak early as mid veigar

cons

• expensive build
• need move speed ASAP
• dependant on team to follow up on your CC