diff --git a/core/src/Boss.cpp b/core/src/Boss.cpp
new file mode 100644
index 0000000..84bc50e
--- /dev/null
+++ b/core/src/Boss.cpp
@@ -0,0 +1,156 @@
+#include "vf/Boss.h"
+
+#include <algorithm>
+#include <cmath>
+#include <utility>
+
+namespace vf {
+
+namespace {
+
+constexpr float fullTurn = 6.2831853f;
+
+float distanceBetween(const Vec3& a, const Vec3& b) { return length(a - b); }
+
+std::unique_ptr<BehaviorNode> cond(const char* name, BehaviorPredicate predicate) {
+    return std::unique_ptr<BehaviorNode>(new ConditionNode(name, std::move(predicate)));
+}
+
+std::unique_ptr<BehaviorNode> act(const char* name, BehaviorAction fn) {
+    return std::unique_ptr<BehaviorNode>(new ActionNode(name, std::move(fn)));
+}
+
+std::unique_ptr<SequenceNode> seq(const char* name) {
+    return std::unique_ptr<SequenceNode>(new SequenceNode(name));
+}
+
+std::unique_ptr<SelectorNode> sel(const char* name) {
+    return std::unique_ptr<SelectorNode>(new SelectorNode(name));
+}
+
+std::unique_ptr<BehaviorNode> gated(const char* name, std::unique_ptr<BehaviorNode> child,
+                                    int ticks) {
+    return std::unique_ptr<BehaviorNode>(new CooldownNode(name, std::move(child), ticks));
+}
+
+std::unique_ptr<BehaviorNode> bounded(const char* name, std::unique_ptr<BehaviorNode> child,
+                                      int ticks) {
+    return std::unique_ptr<BehaviorNode>(new TimeLimitNode(name, std::move(child), ticks));
+}
+
+Vec3 crystalCenter(const Vec3i& cell) {
+    return {static_cast<float>(cell.x) + 0.5f, static_cast<float>(cell.y) + 0.5f,
+            static_cast<float>(cell.z) + 0.5f};
+}
+
+Vec3i witherOffset(const Vec3i& base, int along, int up, bool alongX) {
+    return alongX ? Vec3i{base.x + along, base.y + up, base.z}
+                  : Vec3i{base.x, base.y + up, base.z + along};
+}
+
+} // namespace
+
+const char* bossKindName(BossKind kind) {
+    switch (kind) {
+        case BossKind::EnderDragon: return "enderDragon";
+        case BossKind::Wither: return "wither";
+        case BossKind::Count: break;
+    }
+    return "unknown";
+}
+
+const char* bossPhaseName(BossPhase phase) {
+    switch (phase) {
+        case BossPhase::Dormant: return "dormant";
+        case BossPhase::Circling: return "circling";
+        case BossPhase::Charging: return "charging";
+        case BossPhase::Breathing: return "breathing";
+        case BossPhase::Perching: return "perching";
+        case BossPhase::Summoning: return "summoning";
+        case BossPhase::Explosive: return "explosive";
+        case BossPhase::Ranged: return "ranged";
+        case BossPhase::Armored: return "armored";
+        case BossPhase::Dying: return "dying";
+        case BossPhase::Defeated: return "defeated";
+        case BossPhase::Count: break;
+    }
+    return "unknown";
+}
+
+int BossBar::notchCount() const {
+    switch (style) {
+        case BossBarStyle::Solid: return 1;
+        case BossBarStyle::Notched6: return 6;
+        case BossBarStyle::Notched10: return 10;
+        case BossBarStyle::Notched12: return 12;
+        case BossBarStyle::Notched20: return 20;
+        case BossBarStyle::Count: break;
+    }
+    return 1;
+}
+
+int BossBar::filledNotches() const {
+    float exact = progress * static_cast<float>(notchCount());
+    int whole = static_cast<int>(std::floor(exact + 1e-4f));
+    return std::max(0, std::min(notchCount(), whole));
+}
+
+int BossBar::percent() const {
+    return static_cast<int>(std::floor(progress * 100.0f + 0.5f));
+}
+
+void BossBar::setFromHealth(float health, float maxHealth) {
+    if (maxHealth <= 0.0f) {
+        progress = 0.0f;
+        return;
+    }
+    float fraction = health / maxHealth;
+    progress = std::max(0.0f, std::min(1.0f, fraction));
+}
+
+void BossState::enterPhase(BossPhase next) {
+    if (phase == next) return;
+    phase = next;
+    phaseTicks = 0;
+    ++phaseChanges;
+}
+
+float BossState::healthFraction() const {
+    if (maxHealth <= 0.0f) return 0.0f;
+    return health / maxHealth;
+}
+
+std::vector<CrystalNode> crystalsFromPillars(const std::vector<ObsidianPillar>& pillars) {
+    std::vector<CrystalNode> nodes;
+    nodes.reserve(pillars.size());
+    for (const ObsidianPillar& pillar : pillars) {
+        CrystalNode node;
+        node.pillarBase = pillar.base;
+        node.pillarHeight = pillar.height;
+        node.position = {pillar.base.x, pillar.base.y + pillar.height + 1, pillar.base.z};
+        node.caged = pillar.crowned;
+        node.cageIntact = pillar.crowned;
+        nodes.push_back(node);
+    }
+    return nodes;
+}
+
+DragonFight::DragonFight(const EndGenerator& end, uint32_t seed, const DragonFightConfig& config)
+    : cfg(config) {
+    boss.kind = BossKind::EnderDragon;
+    boss.maxHealth = cfg.maxHealth;
+    boss.health = cfg.maxHealth;
+    boss.rng = seed != 0u ? seed : 0x51ed270bu;
+    boss.phase = BossPhase::Circling;
+    boss.bar.title = "Ender Dragon";
+    boss.bar.color = BossBarColor::Pink;
+    boss.bar.style = BossBarStyle::Notched10;
+    boss.bar.visible = true;
+    boss.bar.setFromHealth(boss.health, boss.maxHealth);
+
+    pillarCrystals = crystalsFromPillars(end.pillars());
+
+    int surface = end.surfaceHeight(0, 0);
+    portal = {0.5f, static_cast<float>(surface), 0.5f};
+    eggCell = {0, surface + 1, 0};
+