Mask of the Master
To get the "Mask of the Master" ('Maske des Meisters', +2 CH) speak to 'Erna Rübenfein' in Avestreu (before you go into Ferdock, next to the soliders at the bridge) and enter one of the following codes: 

3EXD6L
44SC2K
4X1W68
6LAM24
74ZUA1
84JDE9
9PQ1WC
9YF3LC
EFG65K
F4B7AY
F5ZHV7
G97LG1
H6V8WS
IM3Z91
J7FB8T
JK28AX
LK8J4X
MP97FR
N4DEL5
NB3DY6
P38FVQ
PY66M4
RBV38S
S35DF9
S3RBD1
T6FM9Q
TW5BCW
U8IS57
YAPL39
Z75MRG 

Any of the codes should work, but it is confirmed that some of them are locked to a certain version of the game (such as pre-order, collector's edition, Amazon's pre-order edition). To save you time trying to find a correct code for your edition just open the file Drakensang\export\db\static.db4 with an editor and search for "keyword=".