Grimtooth and Sunsword invoke can be paid with magic power

If the invoke timeout hasn't run out, the effect will take 25 pw,
the same as a lvl 5 spell.
If hero doesn't have that much, then the invoke fails.

doc/fixes3-7-0.txt

@@ -1510,6 +1510,7 @@ if a tree and a boulder or statue were at the same location, applying an axe
 	would break the boulder or statue rather than chop the tree
 avoid premapping outside Sokoban map to prevent showing stone glyphs
 Grimtooth is permanently poisoned, protects from poison, and can fling poison
+invoke cost for Grimtooth and Sunsword can be paid with magic power
 cursed magic whistle can teleport you to your pet
 Fire and Frost Brand can be invoked for expert level fireball or cone of cold
 wielding Trollsbane grants hungerless regeneration

src/artifact.c

@@ -40,6 +40,8 @@ staticfn int invoke_banish(struct obj *) NONNULLARG1;
 staticfn int invoke_fling_poison(struct obj *) NONNULLARG1;
 staticfn int invoke_storm_spell(struct obj *) NONNULLARG1;
 staticfn int invoke_blinding_ray(struct obj *) NONNULLARG1;
+staticfn int arti_invoke_cost_pw(struct obj *) NONNULLARG1;
+staticfn boolean arti_invoke_cost(struct obj *) NONNULLARG1;
 staticfn int arti_invoke(struct obj *);
 staticfn boolean Mb_hit(struct monst * magr, struct monst *mdef,
                       struct obj *, int *, int, boolean, char *);
@@ -2083,6 +2085,48 @@ invoke_blinding_ray(struct obj *obj)
     return ECMD_TIME;
 }
 
+/* return the amount of Pw invoking an object costs.
+   return a negative value, if obj invoking cannot be paid with Pw */
+staticfn int
+arti_invoke_cost_pw(struct obj *obj)
+{
+    const struct artifact *oart = get_artifact(obj);
+
+    if (oart->inv_prop == FLING_POISON
+        || oart->inv_prop == BLINDING_RAY) {
+        /* pretend it's a level 5 spell */
+        return SPELL_LEV_PW(5);
+    }
+
+    return -1;
+}
+
+/* return TRUE if artifact object's invoke cost can be paid (and pay it) */
+staticfn boolean
+arti_invoke_cost(struct obj *obj)
+{
+    if (obj->age > svm.moves) {
+        int pw_cost = arti_invoke_cost_pw(obj);
+
+        if (pw_cost < 0 || u.uen < pw_cost) {
+            /* the artifact is tired :-) */
+            You_feel("that %s %s ignoring you.", the(xname(obj)),
+                     otense(obj, "are"));
+            /* and just got more so; patience is essential... */
+            obj->age += (long) d(3, 10);
+            return FALSE;
+        } else {
+            /* you pay invoke cost with your own magic */
+            You_feel("drained...");
+            u.uen -= pw_cost;
+            disp.botl = TRUE;
+        }
+    } else {
+        obj->age = svm.moves + rnz(100);
+    }
+    return TRUE;
+}
+
 staticfn int
 arti_invoke(struct obj *obj)
 {
@@ -2102,17 +2146,10 @@ arti_invoke(struct obj *obj)
         return ECMD_TIME;
     }
 
+    /* It's a special power, not "just" a property */
     if (oart->inv_prop > LAST_PROP) {
-        /* It's a special power, not "just" a property */
+        if (!arti_invoke_cost(obj))
-        if (obj->age > svm.moves) {
-            /* the artifact is tired :-) */
-            You_feel("that %s %s ignoring you.", the(xname(obj)),
-                     otense(obj, "are"));
-            /* and just got more so; patience is essential... */
-            obj->age += (long) d(3, 10);
             return ECMD_TIME;
-        }
-        obj->age = svm.moves + rnz(100);
 
         switch (oart->inv_prop) {
         case TAMING: res = invoke_taming(obj); break;