OVERVIEW
Mindflayer is almost exclusively a hard support. The main choice in his build and playstyle is whether he focusses on Migraine or on his other spells. If Migraine is very valuable this game go the left talent tree and get the shard. Otherwise go the right talent tree.
The item build is incredibly flexible. Buy what is needed. Need to protect teammates? Force, glimmer and pavise. Need to survive? Aeon disk, bkb. Lock down? Euls, atos, or hex. In certain games, where Migraine is super valuable, getting a lot of intelligence can make Mindflayer quite the threat. And if you have that lots of intelligence then the scepter becomes very enticing.