Rework the Horn of Geryon as an xp-charged evocable.

Previously the horn was a nearly useless item after unbarring the hells and
possibly opening a trove.  This version keeps the summon hell beast theme
but turns it into a more useful xp-charging evocable.  Evoking it when it's
charged summons 1-4 hell beasts, depending on evocations, each summon
having a small chance of being hostile, also depending on evocations.  The
chance of getting a hostile hell beast is 7 in evocations * 10 (always
hostile at evocations 0).  Two hell beasts get created 63% of the time at
10 evocations, three 63% of the time at 15 evocations, and 4 66% of the
time at 20 evocations, with always 4 summoned at higher than 23 evocations.

It could perhaps use an additional effect, but it's an always-generated
item that's relatively accessible in a 3-rune game, so some care has to be
taken there.

1 files changed, 3 insertions, 0 deletions

diff --git a/crawl-ref/source/goditem.cc b/crawl-ref/source/goditem.cc
index 6873bb1d73..2a684765e7 100644
--- a/crawl-ref/source/goditem.cc
+++ b/crawl-ref/source/goditem.cc
@@ -112,6 +112,9 @@ bool is_unholy_item(const item_def& item)
     case OBJ_RODS:
         retval = _is_bookrod_type(item, is_unholy_spell);
         break;
+    case OBJ_MISCELLANY:
+        retval = item.sub_type == MISC_HORN_OF_GERYON;
+        break;
     default:
         break;
     }