Looking to get your take on this. I've been operating on the principle that Strategems could only ever be used once per phase in Matched Play. However, RAW it says that Strategems not used during a phase are not affected. So, a Strategem that says "use before the battle" or "during deployment" or "when mustering" are specifically exempt from the once-per-phase rule.
I am specifically curious about the Death Visions of Sanguinius one for BA. As you can see below that it does not specifically say it can only be used once (like the one for taking extra relics)...so by RAW of the Matched Play rules, you could theoretically use it before the battle for as many characters as you have?
The same Strategem cannot be used by the same player more than once during any single phase. This does not affect Stratagems that are not used during a phase, such as those used "before the battle begins" or "at the end of of a battle round."
Use this Strategem when mustering your army. Select a Blood Angels Captain, Chaplain, or Lieutenant from your army (other than Primaris models, and named characters such as Commander Dante). That model gains the Death Company keyword and the Black Rage ability for the duration of the battle.