diff --git a/internal/plugin/zone_graph_sunken_temple.go b/internal/plugin/zone_graph_sunken_temple.go
new file mode 100644
index 0000000..f3891e2
--- /dev/null
+++ b/internal/plugin/zone_graph_sunken_temple.go
@@ -0,0 +1,71 @@
+package plugin
+
+// Phase G8c — Sunken Temple of Dar'eth branching graph.
+//
+// T2 zone. Shape: sequential forks (no mid-path merge). Four distinct
+// paths to the boss; the player commits at fork1 (dry vs. wet) and
+// commits again at fork2a/fork2b. Path is unique up to the boss room.
+//
+//   entry → flooded_atrium → fork1
+//                              ├─[unlocked, w=1]── fork2a (dry)
+//                              │                      ├─[unlocked]── kuo_toa_pen (elite) → boss
+//                              │                      └─[STR DC 13]── glyph_chamber → boss
+//                              └─[Perception DC 13, w=2]── fork2b (wet)
+//                                                     ├─[unlocked]── aboleth_thralls → boss
+//                                                     └─[Perception DC 15]── coral_reliquary (secret) → boss
+//
+// Distinct from the Crypt diamond and the Forest asymmetric-diamond:
+// no two paths share a mid-node, so the !zone map paints two parallel
+// "Y" subtrees instead of a converging branch. The four leaves all
+// terminate at the boss (validator requires exactly one boss node).
+
+func zoneSunkenTempleGraph() ZoneGraph {
+	nodes := []ZoneNode{
+		{NodeID: "sunken_temple.entry", Kind: NodeKindEntry, IsEntry: true,
+			Label: "Tide-Stained Threshold", PosX: 0, PosY: 2},
+		{NodeID: "sunken_temple.flooded_atrium", Kind: NodeKindExploration,
+			Label: "Flooded Atrium", PosX: 1, PosY: 2},
+		{NodeID: "sunken_temple.fork1", Kind: NodeKindFork,
+			Label: "Split Stair", PosX: 2, PosY: 2},
+		{NodeID: "sunken_temple.fork2a", Kind: NodeKindFork,
+			Label: "Dry Crossing", PosX: 3, PosY: 0},
+		{NodeID: "sunken_temple.fork2b", Kind: NodeKindFork,
+			Label: "Submerged Crossing", PosX: 3, PosY: 4},
+		{NodeID: "sunken_temple.kuo_toa_pen", Kind: NodeKindElite,
+			Label: "Kuo-toa Pen", PosX: 4, PosY: 0},
+		{NodeID: "sunken_temple.glyph_chamber", Kind: NodeKindExploration,
+			Label: "Glyph Chamber", PosX: 4, PosY: 1},
+		{NodeID: "sunken_temple.aboleth_thralls", Kind: NodeKindExploration,
+			Label: "Thrall Pool", PosX: 4, PosY: 3},
+		{NodeID: "sunken_temple.coral_reliquary", Kind: NodeKindSecret,
+			Label: "Coral Reliquary", PosX: 4, PosY: 4,
+			Content: ZoneNodeContent{LootBias: 1.8}},
+		{NodeID: "sunken_temple.boss", Kind: NodeKindBoss, IsBoss: true,
+			Label: "Aboleth's Pool", PosX: 5, PosY: 2},
+	}
+	edges := []ZoneEdge{
+		{From: "sunken_temple.entry", To: "sunken_temple.flooded_atrium", Lock: LockNone},
+		{From: "sunken_temple.flooded_atrium", To: "sunken_temple.fork1", Lock: LockNone},
+		{From: "sunken_temple.fork1", To: "sunken_temple.fork2a", Lock: LockNone, Weight: 1},
+		{From: "sunken_temple.fork1", To: "sunken_temple.fork2b",
+			Lock: LockPerception, LockData: map[string]any{"dc": 13},
+			Hint: "wet stone glistens down a side passage", Weight: 2},
+		{From: "sunken_temple.fork2a", To: "sunken_temple.kuo_toa_pen", Lock: LockNone, Weight: 1},
+		{From: "sunken_temple.fork2a", To: "sunken_temple.glyph_chamber",
+			Lock: LockStatCheck, LockData: map[string]any{"stat": "STR", "dc": 13},
+			Hint: "a stone door wedged half-shut by silt", Weight: 2},
+		{From: "sunken_temple.fork2b", To: "sunken_temple.aboleth_thralls", Lock: LockNone, Weight: 1},
+		{From: "sunken_temple.fork2b", To: "sunken_temple.coral_reliquary",
+			Lock: LockPerception, LockData: map[string]any{"dc": 15},
+			Hint: "a coral arch glittering under the surface", Weight: 2},
+		{From: "sunken_temple.kuo_toa_pen", To: "sunken_temple.boss", Lock: LockNone},
+		{From: "sunken_temple.glyph_chamber", To: "sunken_temple.boss", Lock: LockNone},
+		{From: "sunken_temple.aboleth_thralls", To: "sunken_temple.boss", Lock: LockNone},
+		{From: "sunken_temple.coral_reliquary", To: "sunken_temple.boss", Lock: LockNone},
+	}
+	return BuildGraph(ZoneSunkenTemple, nodes, edges)
+}
+
+func init() {
+	registerZoneGraph(zoneSunkenTempleGraph())
+}
diff --git a/internal/plugin/zone_graph_sunken_temple_test.go b/internal/plugin/zone_graph_sunken_temple_test.go
new file mode 100644
index 0000000..329ecf4
--- /dev/null
+++ b/internal/plugin/zone_graph_sunken_temple_test.go
@@ -0,0 +1,80 @@
+package plugin
+
+import "testing"
+
+func TestSunkenTempleGraph_Registered(t *testing.T) {
+	g, ok := zoneGraphRegistry[ZoneSunkenTemple]
+	if !ok {
+		t.Fatal("zoneSunkenTempleGraph not registered")
+	}
+	if g.Entry != "sunken_temple.entry" {
+		t.Errorf("entry node = %q", g.Entry)
+	}
+	if g.Boss != "sunken_temple.boss" {
+		t.Errorf("boss node = %q", g.Boss)
+	}
+	if len(g.Nodes) != 10 {
+		t.Errorf("nodes = %d, want 10", len(g.Nodes))
+	}
+}
+
+// TestSunkenTempleGraph_FourDistinctPaths verifies the four leaves
+// (kuo_toa_pen, glyph_chamber, aboleth_thralls, coral_reliquary) all
+// reach the boss. Together with the no-merge invariant below, this
+// confirms the design intent: four committed paths, not a diamond.
+func TestSunkenTempleGraph_FourDistinctPaths(t *testing.T) {
+	g := zoneSunkenTempleGraph()
+	for _, leaf := range []string{
+		"sunken_temple.kuo_toa_pen",
+		"sunken_temple.glyph_chamber",
+		"sunken_temple.aboleth_thralls",
+		"sunken_temple.coral_reliquary",
+	} {
+		if !reachable(g, leaf, "sunken_temple.boss") {
+			t.Errorf("%s unreachable to boss", leaf)
+		}
+	}
+}
+
+// TestSunkenTempleGraph_NoMidPathMerge ensures no node other than the
+// boss has more than one incoming edge. If a future edit collapses the
+// two subtrees into a converging diamond, this test fails — that's the
+// signal to either rename the zone's shape comment or re-author.
+func TestSunkenTempleGraph_NoMidPathMerge(t *testing.T) {
+	g := zoneSunkenTempleGraph()
+	incoming := map[string]int{}
+	for _, outs := range g.Edges {
+		for _, e := range outs {
+			incoming[e.To]++
+		}
+	}
+	for id, n := range g.Nodes {
+		if n.IsBoss {
+			continue
+		}
+		if incoming[id] > 1 {
+			t.Errorf("node %q has %d incoming edges, want ≤1 (no mid-path merge)", id, incoming[id])
+		}
+	}
+	// Boss intentionally has 4 incoming edges (one per leaf).
+	if incoming["sunken_temple.boss"] != 4 {
+		t.Errorf("boss incoming = %d, want 4", incoming["sunken_temple.boss"])
+	}
+}
+
+func TestSunkenTempleGraph_LockKindCoverage(t *testing.T) {
+	g := zoneSunkenTempleGraph()
+	kinds := map[ZoneEdgeLockKind]bool{}
+	for _, outs := range g.Edges {
+		for _, e := range outs {
+			kinds[e.Lock] = true
+		}
+	}
+	// Sunken Temple deliberately exercises Perception + StatCheck +
+	// LockNone in one zone.
+	for _, k := range []ZoneEdgeLockKind{LockNone, LockPerception, LockStatCheck} {
+		if !kinds[k] {
+			t.Errorf("missing lock kind %s — variety check", k)
+		}
+	}
+}