I put holy word in my clerics book thinkin it'll be a good spell. When I cast it against undead or evil characters is does nothing at all. A couple of birds fly away from me but it has no effect on the enemy. Whats it supposed to do?.

Warby

Level 1-3: Death
Level 4-7: Stunned for 1 turn
Level 8-11: Slowed for 1 turn with 75% chance of spell failure
Level 12+: Deafened for 1 turn with 50% chance of spell failure

Since this is ToB we are talking about, where everybody and his pet cat is level 12+, I would memorise Sunray instead to use against Undead and Summon Deva or Implosion for anything else.