role: usuelly mid, can offlane if it is not so hard or with a support, can go carry but not enough alone to carry a game, and can go support if he have good carry who will get use of his empower.
best combos: since he has a good spell to gather all the enemies in one place , all battlefury carries and aoe spells will be very good with him.
counters: magnus needs the enemies to be close together so he can ulti them, so a diversity of ranges and melee will be good to disable his RP (sniper, huskar, TA, ...etc) and heroes who can counter the ulti or make it useless (huskar, phoenix, .... etc).
gathering the enemies which can be extremely useful with a good combo
his empower gives huge amount of damage to teammates
very good initiator, and if initiated well he can grantee a won fight.
low mana while needs a lot of it
not easy to start a fight with and can make disasters if fails