Valerius Maior, Hell's Fool

Timo wrote:

I asked myself this same question and I thought about the +1 bleed modifier or +1 stealth/+1 intercept.

None of them says "until end of action" but it is still the case.

Rulebook tells action modifier effect lasts until the end of the action.

Timo wrote:

The effect lasts for as long as the target is the acting vampire

If effect is limited by acting minion being acting, then why doesn't it limited by Valerius being blocked? Valerius special has two requirements: him attempting to block (this is temporary status) and target being acting (also temporary status). Then why the first temporary status (attempting to block) duration does not matter (trigger once work even if status is not presented anymore), but for some reason second temporary status (being active) matters and remove the effect?

I completely get the idea that this may be how the card is intended to work. My take is that card text does not reflect it in the clear way (to be read from card text and rulebook).

That the text could be more elaborate and more explicit it always can, but imo that doesn´t mean it isn´t clear as it currently is worded.

If Valerius attempts to block, the acting minion cannot play action modifier or combat cards that require Chimerstry or Obfuscate .

"If Valerius attempts to block" = condition when the effect would trigger, it is not a continues check kind of activation as for that it would need to be worded eg "while Valerius attempts to block"

"the acting minion cannot play action modifier or combat cards that require Chimerstry or Obfuscate ." = the actual effect (that is a modifier type effect) applied on the acting minion, like all modifier effects they only last for the duration of the action that is being affected if not explicitly stated otherwise.

inm8 wrote:

that is a modifier type effect) applied on the acting minion, like all modifier effects

That is not modifier type effect. That is effect without specified duration and that is why I assume it duration must be either limited by duration of triggering condition ("while Valerius is attempting to block") or it will last as long as we want it to be (end of action, turn, game...).
See Arnulf text which is similar:
: If Arnulf attempts to block, the acting minion cannot play action modifier cards requiring Dementation, Dominate, Necromancy or Presence
until the end of the action.

Whether you agree with it being a modifier type effect or not doesn´t change that it is, this because it affects the action as a whole and what can be done during the action by the acting and non-acting, the effect is similar to/no different than making an action eg unblockable.

Here you have a ruling that makes it clear that modifier type effect can be sourced from other card types than action modifiers.
www.vekn.net/forum/rules-questions/80984-haqim-s-law-retribution-efect-during-combat?start=12#109917

With regards to the wording that you provide ("until the end of the action.") is part of the newer templating that has been added to be more explicit about how it works but is in no way changing the effect which was rules the same by LSJ,back at 2005, with the same reasoning that the card text doesn't state "while Valerius is attempting to block".
groups.google.com/g/rec.games.trading-cards.jyhad/c/vet0HGRxtXQ/m/gRJalPv4lLoJ

Below is a card that uses the "while" format and functions as you are expecting.