Revamp shadows; move them deeper in the dungeon

Instead of being irrelevant and unthreatening to virtually every
character, give shadows the ability to strike from, er, the shadows.

Shadows can now turn invisible for brief periods of time. While
invisible, their movement speed is increased, but their melee attack
in particular gets a special property. When a shadow attacks a foe who
cannot see it, the attack counts as a stab (250% normal damage) and
becomes AUTOMATIC_HIT. The shadow is automatically revealed after
performing this attack. Shadows get no movement bonus if they are
backlit, and their attack has no special bonus against a target that
can see invisible (though hitting it will still reveal the shadow to
other creatures). At the same time, this commit also boosts their base
attack to 14 (thus 35 on a successful stab). They do still have fairly
low hp, however (but possibly their AC should also be lowered?)

While unseen horrors already provide the threat of an invisible melee
monster at similar depths, I hope that the fact that the shadow cannot
surprise you in the same way (being visible when first seen and having
predictable movement), yet is still more threatening to allow to hit you
while invisible, will necessitate different tactics when handling them.
Also, it should offer another viable lower-tier monster for Crypt.

1 files changed, 2 insertions, 1 deletions

diff --git a/crawl-ref/source/mon-enum.h b/crawl-ref/source/mon-enum.h
index 25b7a9cc44..5b78ad577f 100644
--- a/crawl-ref/source/mon-enum.h
+++ b/crawl-ref/source/mon-enum.h
@@ -112,7 +112,8 @@ enum attack_flavour
     AF_DRAIN_SPEED,
     AF_VULN,
     AF_PLAGUE,
-    AF_WEAKNESS_POISON
+    AF_WEAKNESS_POISON,
+    AF_SHADOWSTAB,
 };
 
 // Non-spell "summoning" types to give to monster::mark_summoned(), or