Goto Guildmaster in Silvermere
Greet guildmaster
Ask guildmaster call to arms
Ask guildmaster orc

Goto Slavemaster Area in Huge Cave
Twist skull in bedroom
Find the Orc Warlord and kill him
Get his head

Goto Guildmaster in Silvermere
Give head of orc warlord to guildmaster

You have completed the Guildmaster's quest, you will get 1000 exp and 500 gold!