Make draining temporary (for monsters)

The hit dice-reduction effect of draining has historically had
several problems. It reduced monsters' maximum hp, which made it
look like they were getting *less* injured, since they had a
higher proportion of hp remaining. It lowered monster XP & piety
gains, which was irrelevant but misled new players who somehow
learned about it. It occasionally led to "degenerate" hit-and-run
tactics.

And most damningly of all, it hardly ever mattered - it
triggered on ~13% of hits, which meant that on low HD monsters
the extra damage would kill them before the effect was
noticeable, and against high HD monsters, the effect would only
ever be noticeable at all with the aforementioned hit-and-run
tactics.

So, to fix those problems, draining now gives a "drained" status,
that reduces monster HD for most combat-related purposes
(spellcasting, accuracy, damage, etc.), but not max hp, xp, or
piety. This is temporary, but will last 20-30 turns, and
refreshes every time the drain triggers - essentially, it should
last until you kill the monster, unless you run away.

The temp-status is now applied to the monster every time they get
drained; the chance of the drain brand activating has been reduced
to 1/2, from 2/3. This should focus the effects of the brand more
on the unique part of it, the draining/weakening effect.

As a bonus, this also means that players can no longer have their
followers permanently weakened by draining effects. Beogh buff!

1 files changed, 6 insertions, 9 deletions

diff --git a/crawl-ref/source/monster.cc b/crawl-ref/source/monster.cc
index cab7b88615..3d90ff9c81 100644
--- a/crawl-ref/source/monster.cc
+++ b/crawl-ref/source/monster.cc
@@ -4247,15 +4247,12 @@ bool monster::drain_exp(actor *agent, bool quiet, int pow)
 
     if (alive())
     {
-        if (one_chance_in(5))
-        {
-            if (hit_dice > 1)
-                hit_dice--;
-            experience = 0;
-        }
-
-        max_hit_points -= 2 + random2(3);
-        hit_points = min(max_hit_points, hit_points);
+        const int dur = min(200 + random2(100),
+                            300 - get_ench(ENCH_DRAINED).duration
+                                - random2(50));
+        const mon_enchant drain_ench = mon_enchant(ENCH_DRAINED, 1, agent,
+                                                   dur);
+        add_ench(drain_ench);
     }
 
     return true;