Effects

There are five different ones in the game - 5 unique holy grails. One serves for each mage colour. So you only have use for one of the grails.

It will resurrect up to 15% of dead units, as long as at least one unit of the stack survives the battle.
Only 40% of the Units with the Healing Ability are resurrected (30% Healing + 10% from the Holy Grail)
Hidden Upkeep: it reduces your mana production by 1000 mp on Arch, and 500 on ultimate servers.

Demystification

1. there are a few grails, and only one works for you, some people say one colour of grail is the only real one, some people say that there is a real one for each colour (eg the copper grail works for X colour) Some people are wrong.
2. when you used your map it took the grail off someone else
3. when someone else used a map they took it off you
4. The real grail is a good item, but because of all the maps it does not last long, therefore while other uniques sell for hundreds of millions plus plus, the grails are only bought of sold for loose change. You can use one for maybe a few hours, and then it will go.
5. It is nice to have during an attack run, or on defence.
6. Other uniques will, on average, last for weeks or even the whole reset.
7. It is rumored to increase your recruit rate.

Holy Grails disappear whenever someone successfully uses Blood Stained Map and wins your grail (1/5 chance, happens mostly within 12-24 hours).
Holy Grails can also be obtained from Treasure Maps and Treasure Chests.
Buy - use immedeatly after arrival - sell is a common strategy of working with Holy Grails.
- Five grails coexist in the game. But when they are sold on the Black Market, it can happen that a second copy gets created through a Blood Stained Map. Soon as the Black Market one is sold, and re-enters the game, the second one disappears.

It is rumored to increase Knight's Attack Power. FALSE - AP was the same as normal (250 as of writing).
It is rumored to double the Knights gained from Horn of Valhalla. FALSE - Knights gained was 38xx, the normal gained.