The bonuses you get depend on your class. There are different versions on how exactly bonuses work. These are 2 versions I have stolen from the internet :P :
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1.
http://forum.jowood.com/archive/index.php/t-111416.html - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Praying in NotR: only one time each day - but unlimited
guildless: 10 times +1 strength, 10 times +1 dexterity, 20 times +1 mana, then always only 3 life points (unlimited)
if you become then a mage you can´t get strenght and dex anymore. As dragon hunter you can´t get mana anymore.
Novice/Mage: 20 times +1 mana, then always only 3 life points (unlimited)
Mercenary/Dragon Hunter: 10 times +1 strength, 10 times +1 dexterity, then always only 3 life points (unlimited)
Militia/Paladin: 10 times +1 strength, 10 times +1 dexterity, 20 times +1 mana, then always only 3 life points (unlimited)
If you spend 50 Gold you will get 2 life points and your life will be full regenerated.
If you spend 10 Gold you will get 1 life points and your life will be full regenerated.
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2.
http://www.gamefaqs.com/computer/doswin/file/920919/45017 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
You can pray at a Shrine of Innos once a day and recieve bonuses for donating
100g. You are limited to 10 bonuses for each stat increase (you can get 10 +1
STR, 10 +1 DEX, for example), any more than that and there is no reward.
Possible Rewards: +1 STR, +1 DEX, +4 HP, +2 MP
There seems to be an influence of what you get based on your guild and/or which
ever stat you have more/less of (DEX versus STR), however I am not sure of the
exact probabilty and formula. As a mage you get Mana bonuses primarily. Merc
gets HP and STR, Paladins get double bonuses for praying. That being said, you
just need to find out for yourself.
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There may be differences between original Gothic 2 and Night of the Raven, and the fact that you are getting only dex is probably related to your current statistics.
About artefacts increasing your statistics take a look here:
http://www.worldofgothic.com/gothic2/?go=g2ringe You can use Amulet of Strength or Amulet of Power.
(That site is a perfect place to look for hints and maps by the way).
A way to permanently increase your strength is a potion of strength. These are rare, but you can learn a recipe from Constantino (if you work for him).
