Adjust Demonbane invoke ability

Demon lords and princes have a chance to resist the effect.
Demons in quest when nemesis is alive have a very high chance
of resisting.
When invoked in Gehennom, teleports the demons within the same
level.

src/artifact.c

@@ -1832,14 +1832,15 @@ arti_invoke(struct obj *obj)
             break;
         }
         case BANISH: {
-            if (!Inhell) {
+            int nvanished = 0, nstayed = 0;
-                int nvanished = 0;
             struct monst *mtmp, *mtmp2;
             d_level dest;
 
             find_hell(&dest);
 
             for (mtmp = fmon; mtmp; mtmp = mtmp2) {
+                int chance = 1;
+
                 mtmp2 = mtmp->nmon;
                 if (DEADMONSTER(mtmp) || !isok(mtmp->mx, mtmp->my))
                     continue;
@@ -1847,14 +1848,34 @@ arti_invoke(struct obj *obj)
                     continue;
                 if (!couldsee(mtmp->mx, mtmp->my))
                     continue;
-                    dest.dlevel = rn2(dunlevs_in_dungeon(&dest));
+                if (mtmp->data->msound == MS_NEMESIS)
+                    continue;
+
+                if (In_quest(&u.uz) && !g.quest_status.killed_nemesis)
+                    chance += 10;
+                if (is_dprince(mtmp->data))
+                    chance += 2;
+                if (is_dlord(mtmp->data))
+                    chance++;
+
                 mtmp->msleeping = mtmp->mtame = mtmp->mpeaceful = 0;
+                if (chance <= 1 || !rn2(chance)) {
+                    if (!Inhell) {
                         nvanished++;
+                        /* banish to a random level in Gehennom */
+                        dest.dlevel = rn2(dunlevs_in_dungeon(&dest));
                         migrate_mon(mtmp, ledger_no(&dest), MIGR_RANDOM);
+                    } else {
+                        u_teleport_mon(mtmp, FALSE);
+                    }
+                } else {
+                    nstayed++;
+                }
             }
 
-                if (nvanished)
+            if (nvanished) {
-                    pline("The demon%s disappear%s in a cloud of brimstone!",
+                pline("%she demon%s disappear%s in a cloud of brimstone!",
+                      nstayed ? (nvanished > nstayed ? "Most of t" : "Some of t") : "T",
                       nvanished > 1 ? "s" : "",
                       nvanished > 1 ? "" : "s");
             }