Draykavac is seemingly an odd entry in the Questoris Knight army list. He is a traitor Thagmata entry who rose from relative obscurity to become a truly evil piece of work in the era of the Heresy.
The High Techno-Arcana special rule is probably what makes this Archmagos suitable for this army. In brief, he provides a +1 to charge distances and sweeping advance rolls. This can be a good boon, but considering that some knights can run 3d6, its arguable if this is so great.
The warlord trait he comes with automatically is sound, with preferred enemy when holding objectives against infantry. This clearly calls for a semi-defensive build of knights who are aiming to go for capturing and holding objectives, rather than an all-out alpha strike to remove every enemy from the board at all costs. Then again, is your enemy going to be primarily infantry based or not? It could be very tough if the enemy is yet more knights. On top of that, not every mission has objectives. Just saying.
The Liquifactor rule provides a neat way to turn an enemy in to, well, a liquid. Its strong in close combat, if a little unusual.
In terms of game stats, the one thing that stands out is T=6 which is outstanding for a mechanicum infantry. His 2+ save also helps out immensely with survivability. He can also help repair knights thanks to being a battle smith.
He has a relatively weak strength value and only 2 attacks. But this is not the reason why you are taking this character.
Overall, I'm not sure whether to like this HQ choice or not. The points cost is very steep. And he doesn't exactly fit in well with Questoris list which presumably would want knights and more knights. He's better saved for a Taghmata HQ. Equally, the ability to take a unit of Castellax or Vorax as fast attack choices could be very useful and provide a much needed alternative to more knights, if desired. Equally, if I'm playing Questoris knights, I'd personally rather like to play knights and more knights. But that's just me.