quick review of his abilities:
passive he gains stacks blah blah
Q channels and deals damage that splashes, "crit"s for every second channeled into a single enemy and gathering stacks
well for critting you need your enemy to stand right there. which one is better enemy minions who die easily and lane opponent who has a brain and wont allow you to crit Q easily, or mindless jungle monsters who just sit there and attack you?
W is dash over walls that resets on kill
well basically his ganking ability, which allows him to easily gank and E then Q on the enemy to eat them
E is blackhole drags in middle deals dmg and executes
its probably gonna use kinematics (the mechanic rell ult uses) that will be overridden by enemy movement abilities
its a good ability to start in lane lets you stack passive early and harass but in jungle it can stack up jungle enemies away from you and allow you to safely melt them with Q while executing enemies inside and granting you stacks. in fights it keeps people from running away by walking
and R is star that stuns and becomes empowered that knocks up and deals dmg in insanely large area
also for his build
mythic probably everfrost which gives him AP haste health and mana and allows him to slow/root people to melt them with Q
rocketbelt also gives him additional mobility and nice stats
lindary's and luden's are good items on every mage basically
ROA can give him insane scaling too
abyssal mask can be viable on him for tanky build
demonic for tanky build again
deathcap basically mage snowball item
rylai's it can be great on him bcuz u can keep Q on them easier E pulls them easier and R+ shockwave slows
shadowflame isi another standard mage item
same for void staff
cosmic drive CAN be good it depends on the champ details
gimme your opinions