One of the players in my group wants to "upgrade" his bolter to a daemon weapon, using the nurgle daemon they encountered during Broken Chains. I've read through the rules but I have trouble understanding it all. This is how I interpreted it. First you summon the daemon with the necessary ritual and then you make a Daemonic mastery test to inject it into the item? If one succeds, what decides how many times one can roll one the charts? I guess the binding strength decides but here's the problem. The book have an example where a bloodletter have been bound. It have a total binding strength of 1 and a WP bonus of 3, therefore will roll two times. So…. WP bonus - Binding strength = number of times rolling? If so, then the character who wants to make a daemon weapon will be rolling many times. Lurgach, the Nurgle daemon, have a willpower of 50. So if There will be a DoS of two or one, does that mean he will then be required to roll three or even FOUR times!? + infamy of 25 will result in a great bolter. Should I take away the infamy score or deliberatly lower the Wp?
Firstly, you indeed summon the Daemon, likely via a summoning ritual like those detailed on page 229 and 230. To control it, the summoner makes a Daemonic Mastery Test.
Secondly, you enact another ritual, Forge the Accursed Weapon (page 200). This involves no Daemonic Mastery Test yet, but the test described in the ritual text (Forbidden Lore Warp or Daemonology, starting at -60). You are then correct in that the number of ability rolls are depending on the WP bonus minus Binding strength, although only one of those rolls has the Infamy score added to it - so only one of its abilities has a chance to be, for example, Stream of Corruption or Pestilent Stench. Of course, once the Daemon Weapon has been created, the prospective wielder must indeed Daemonically Master its occupant (though this might be redundant if the wielder is also the summoner of the daemon that initially cowed it in the first ritual).
Sounds like you'd like as little degrees of success on the ritual as possible? Initially, yes, but there's a catch: Every point of binding strength (which makes the weapon weaker ability-wise) makes the daemon suffer a penalty of 5 points on daemonic mastery tests. Now consider that you have to roll Daemonic Mastery against the bound daemon every single time the character is stunned, rendered unconscious or suffers enough WP damage that his WP falls below that of the daemon. Sure, take that once-bound bolter. And have lots of infamy points ready to reroll daemonic mastery so you don't get posessed (see page 142) by your own weapon that will roll its 45 remaining Willpower against yours whenever it senses a weakness…
(Also note that infamy points of Nurgle-aligned PCs can't be used to reroll tests… )
Also, the characteristic damage everyone will suffer in the aftermath, is it permanent? 2D10 damage to all if I'm not mistaken. What reason to have a daemonic bolter when you can't even hit something with it. Must the summoner spend exp later on to replenish his lost strength, agility, fellowship and the rest or will they all regenerate slowly over time?
Unless the characteristic damage is explicitly called "permanent", it never is. You regenerate one point of a trait per day.
(That also means you should not touch the weapon until your WP has regenerated to semi-safe levels!)