The Blood Shrine offers three rites, and every one of them is paid for out of your own body rather than your purse. That makes it the one place in the game where the cost is measured in the resource you were about to spend anyway, which is exactly what makes the timing worth thinking about. It stands in every city, so reaching it is never the problem; the cooldowns are.
An offering costs 40% of your maximum health and returns a 25% faster energy regeneration for 60 minutes, on a 6-hour cooldown.
Read that trade carefully, because it is not a power buff: it is a rate buff. The offering does not give you energy, it makes energy arrive faster for an hour. Its value therefore depends entirely on whether you are going to be spending energy during that hour. Making an offering and then logging off wastes the entire cost, and the cooldown means you cannot simply try again.
The health cost is also a real risk, not just a number. Going into a fight or a risky action at 60% health after an offering is how players lose runs they would otherwise have survived. Offer at the START of an active session, when you are safe and about to spend heavily, and heal before you fight.
Shadow Mending converts mana into health at 1 mana per point of health, up to 50% of your maximum health per rite, on a 30-minute cooldown.
This is the cheapest of the three rites in practice, because mana is the resource most characters have least use for moment to moment. If you are not running magic-heavy actions, mana sitting at full is idle capacity, and Shadow Mending is the way to turn it into something.
The per-rite ceiling and the cooldown together mean it cannot replace potions in an emergency. Treat it as scheduled maintenance between fights rather than as a heal you can rely on mid-crisis.
Witchfire Kindling is Shadow Mending run backwards: it spends health to restore mana, at 1 health per point of mana, up to 50% of your maximum mana per rite, on its own 30-minute cooldown.
The cooldown being its OWN cooldown is the part worth knowing. Kindling and Shadow Mending do not share a timer, so using one leaves the other available, and a character who is short on mana but has health to spare can convert immediately rather than waiting out a rite they did not use.
As with the offering, the health is spent up front and only heals back at the ordinary rate, so this is a rite for the moment before a mana-hungry run rather than something to reach for mid-fight. Both rites are capped by the deficit as well as the ceiling: at near-full mana you are billed only for the little you were actually missing.
The offering sits on a 6-hour cooldown, Shadow Mending on 30 minutes, and Witchfire Kindling on a separate 30 minutes of its own. Those numbers are what stop the Shrine becoming a treadmill, and the offering is very different in character from the other two.
The mending cooldown is short enough to be part of a routine: you can expect to use it several times across an evening. The offering cooldown is long enough that you get at most a few per day, which means each one is a decision about which hour of your day is worth accelerating. If you make an offering carelessly you have not lost much health, but you have lost the hour you would have wanted it for later.