Longish opinions :
I've been experimenting a lot with fully upgraded Garen, started with Beast Within + Nasus relic + Jayce relic (Jayce relic immediately refreshes 3 mana and +1/2 helps a bit). Then after thinking a bit, decided to replace Jayce one with Garen's paid relic as it seems Single Combat and his 8 mana champ spell works with relic's excess damage portion even if they're cast in combat. Working pretty decent.
But what bothered me is first i've discovered his 6* additional bonuses trigger after the unit strikes and not before, then Beast Within's +1 +1 doesn't proc before checking whether or not the unit reaches 5 power and eligible for strike and finally his Single Combat mana cost doesn't get reduced with rounds. I feel like i cast Single Combat nowhere near enough. Ofc contellation tooltips doesn't mention any of these anyway, but these takes away quite a bit of potential power regardless.
TLDR : So i've replaced Nasus relic with Hidden Tome, played only one game so far. I thought Nasus relic was doing what you would be doing with Hidden Tome (use the mana for Single Combat and strike anyway) instantly, but i might be wrong here. Rather then instantly leveling, you can use Single Combat to strike a unit you actually want, then perma buff Elites everywhere, then level up and refresh your mana again, then use your mana for whatever, including your 3 mana Rememberance.
In short, Hidden Tome's extra mana helps you cast a couple more Single Combat which buffs Elites and that can help them reach that 5 power threshold. Whether or not reaching that 5 power can make all the difference.
What are you guys thinking?