add purge spell scene and effect script

scenes/purge.tscn

@@ -0,0 +1,10 @@
+[gd_scene load_steps=3 format=3]
+
+[ext_resource type="PackedScene" path="res://scenes/summon_circle.tscn" id="1_summon"]
+[ext_resource type="Script" path="res://scripts/purge.gd" id="2_script"]
+
+[node name="Purge" type="Node2D"]
+script = ExtResource("2_script")
+
+[node name="SummonCircle" parent="." instance=ExtResource("1_summon")]
+position = Vector2(0, 8)

scripts/purge.gd

@@ -0,0 +1,42 @@
+extends Node2D
+
+const ELITE_DAMAGE_PCT := 0.75
+const FLASH_FADE_DURATION := 0.6
+
+@onready var summon_circle: AnimatedSprite2D = $SummonCircle/AnimatedSprite2D
+
+func _ready() -> void:
+	summon_circle.sprite_frames = summon_circle.sprite_frames.duplicate()
+	summon_circle.sprite_frames.set_animation_loop("default", false)
+	summon_circle.animation_finished.connect(_unleash)
+	summon_circle.play("default")
+
+func _unleash() -> void:
+	var camera = get_node("/root/Game/Camera2D")
+	camera.shake(0.5, 1.5)
+
+	for enemy in get_tree().get_nodes_in_group("enemies"):
+		if not is_instance_valid(enemy):
+			continue
+		if enemy.scale.x > 1.0:
+			enemy.take_damage(int(enemy.max_hp * ELITE_DAMAGE_PCT))
+		else:
+			enemy.take_damage(enemy.hp)
+
+	_flash()
+
+func _flash() -> void:
+	var layer = CanvasLayer.new()
+	layer.layer = 100
+	add_child(layer)
+
+	var rect = ColorRect.new()
+	rect.color = Color(1, 1, 1, 1)
+	rect.set_anchors_preset(Control.PRESET_FULL_RECT)
+	rect.mouse_filter = Control.MOUSE_FILTER_IGNORE
+	layer.add_child(rect)
+
+	var tween = create_tween()
+	tween.tween_property(rect, "color:a", 0.0, FLASH_FADE_DURATION)
+	await tween.finished
+	queue_free()