So, do you think it can be a good idea to use an Apostate Cardinal and his Pontifex Guards as a Magus and his bodyguard ? What changes should be made in your opinion (give the Cardinal some psy powers is the most obvious for me) ?

I would rather take the Cult Magus from Dark Heresy. Up S and T and maybe Wounds by 10, increase psy rating to 3 or 4 and hand-out nid psy powers, especially Hypnotic Gaze. Also, Lightning Reflexes and Tyranid talents/traits.

The Cult Magus itself isn't much of a threat unless he's surrounded by bodyguards who make hitting him hard and dodge into attacks directed at him, while the kill-team has to deal with purestrain genestealers. That will give him the time/space to let of some of his psy powers. Otherwise he's too squishy to be a threat to Space Marines.

Alex

If I remember correctly Genestealer Cult magus were very powerful telepathically. I seem to remember in one story a magus being able to take control of an Arbites Marshall (who would undoubtably have a fair amount of Wp maybe even Resistance (Psychic powers)) over some distance. So I would have his powers focused on mind control. This would be particularly horrific for Astartes if they realised they were starting to be dominated by a xenos witch!

Having him guarded by Pontifex Guards would certainly make sense to represent well trained Brood Brothers, however he is just as likely to be guarded by Hybrids or Purestrain genestealers. In the case of the Hybrids you can fudge this a bit by just giving the Pontifex guards a few extra wounds maybe a few points in toughness and perhaps the multiple arms and fear trait.

There would be an interesting dynamic here though because the Magus isn't the very head of the cult the Patriarch is. Therefore a magus himself will be willing to sacrafice his own life to save the Patriarch. However if the Patriarch is killed before the magus it is cruicial to the survivial of the cult that the magus survives. He will almost certainly flee under the cover of any cult members.

Facing a genestealer cult can be a terrifying prospect for Imperial servants. Not only are you facing a combination of tyranid shock troops, psykers and effectively mutants (hybrids) but you can also be facing Guard level opponents with associated weapons and vehicles.

Finally depending on the influence of the cult it can be difficult to distinguish to differentiate between friend or foe particularly as some of the cults allies may have been foolishly duped into following the xenos group not realising who they are really fighting alongside.

Presumably by the time the Deathwatch are called in the Ordo Xenos have some inkling of the threat they are facing or indeed are facing a full blown cult insurrection. What is the situation in your game?

(BTW I used a Genestealer cult in my DH game although it was part of a wider consipracy involving an Inquisitor who was dabbling in xenos tech and was studying them).

You could have the opening moves of the insurrection be quite confused, for example a PDF unit decalre a noble house heretical and attack them. Because PDF inititally seem to have a good case then Arbites support them. Turns out the PDF are cult troops, Arbites have been tricked and noble house represents the greatest threat to the cult because of their household troops.

Incidentally the noble family may in fact have some slightly dodgy beliefs with regard to the Imperial Creed which the Astartes may not appreciate the subtelty of. The nobles may even worship the Emperor as an ancestor rather than a god in the same manner as the marines..