The Magus is a latter generation hybrid who is possessed of strong psychic powers. He has been in the canon since the earliest days of genestealer cults and is a mainstay for the force. Here in the modern edition, the Magus is illustrated as a psyker and leader in his own right. The modern take is therefore very much in line with what the figure should be: shrouded in mystery and masterful of the darker arts.
The most obvious strength of the Magus is his (her?) psychic powers. With access to the same disciplines as the Patriarch, it is almost a given that the Magus should be upgraded to a level 2 psyker, and should almost always be played alongside a Patriarch in order to dominate the psychic phase as much as possible.
As with other leaders he projects a bubble: in this case adamantium will. This is a good boon, but maybe situational.
Finally, the Magus gains access to the sacred relics of the cults. Some are good, others are a bit not so good.
Naturally, in the equipment department he comes with a force stave which will slowly but surely slay the opposition.
As with all of the other latter generation hybrids, the Magus features a predominantly human statistics line. This renders him very vulnerable. However, the automatic passing of "Look Our Sir!" with the unquestioning loyalty special rule means that when embedded in a large squad, he will survive a long time. Indeed, he has the potential to be a real unit slayer with the force stave in combination with the unquestioning loyalty rule.
Overall, the Magus is really a quality denial unit, a decent tar pit when combined with large units, and one that can take out opponents given time and space. In terms of builds, it should be automatic to take a second mastery level. However, the other options can be interesting. Here are two builds to consider.
Magus with level 2 mastery (65 points).
This is what I would consider the base line.
Magus, level 2 mastery, relic: staff of the subterranean master (85 points).
Provides a nice bit of firepower which will come in handy against monsters.
Magus, level 2 master, relic: the crouchling, two familiars (95 points).
The Crouchling is a great option for the Magus as it generates a bonus power without increasing the mastery level. It provides extra attacks in close combat and with 2 further familiar, this makes the Magus strong in the mind and strong on the attack.