《異形戰士》（Alien Soldier）是 Treasure 在1995年推出、由SEGA發行於Mega Drive的跑動射擊遊戲。原本僅以卡匣形式在日本與PAL地區發售，而北美玩家只能透過Sega Channel線上服務取得，因此在當時雖然話題性十足，但卻鮮少被大眾玩家接觸。然而，正是這種獨特的命運，使《異形戰士》成為硬核玩家之間口耳相傳的「傳奇之作」。

以名為「西耶拉」（Sierra）的星球為舞臺，故事講述恐怖組織Scarlet在名為Xi-Tiger的殘忍怪物帶領下，企圖消滅整個人類世界。主角Epsilon-Eagle原本也是Scarlet的領袖，卻在組織內部的權力鬥爭中身受重傷，被捲入時空裂縫後消失無蹤。當Xi-Tiger奪權後，他把Scarlet引向更極端的暴虐統治，也因此激起內部叛亂，渴望真正的領袖回歸。

在另一邊，一名擁有異能的少年體內寄宿著Epsilon-Eagle的意識，在Xi-Tiger攻擊研究機構、挾持少女時，少年因憤怒而覺醒並變回Epsilon本人。故事至此正式展開——帶著複雜的過去、被逼迫的復活與強烈復仇動機的Epsilon再度降臨，只為追殺背叛他的Xi-Tiger，並終結恐怖組織的支配。

雖然設定冗長又帶點混亂，但遊戲開始後便不再同玩家交代任何劇情。整體節奏以「Boss Rush」級別著稱。有25個關卡與多達26隻以上的頭目，每關的雜兵階段短得近乎象徵性，只是為讓玩家在迎接下一場激烈的戰鬥前稍作緩和。真正的重點在於超高速、極精準、無容錯的Boss對戰，而Treasure在前作《Gunstar Heroes》中的強項，在本作中被完全放大、深化並帶上純粹化的極端特色。

主角Epsilon-Eagle的可動性極其誇張，他能奔跑、滑行、雙重跳躍、滑翔、貼天花板行走，甚至擁有一擊必殺般的高速突進技能「鳳凰衝擊」（Phoenix Force）。只要在滿血狀態下施放突進，整個畫面會瞬間化為爆炸性的破壞力，而這項技巧也成為許多高手破關影片中不可或缺的核心。

遊戲的最大特色之一是六種武器類型與完全自由的武器組合系統。在開局前要自行決定四種武器搭配，而每把武器都有獨立的能量條與彈藥回復方式。更重要的是，還能使用極難掌握的「子彈反彈」技術，只要判斷精準，就能將敵彈轉化為補血，甚至反射回去造成傷害。固定射擊與自由射擊的模式切換，也讓遊戲成為當時 Mega Drive 上少數需要「格鬥遊戲級操作深度」的射擊作品。

《異形戰士》的開發歷史同樣充滿傳奇色彩。Treasure在完成《Gunstar Heroes》後同時啟動四款新作，其中《異形戰士》由菅波秀幸領軍。他原本打算「一個人做完整款遊戲」，打造出遠超世代的龐大內容與複雜機制。然而當時新世代32位機正在興起，Mega Drive的市場急速萎縮，他不得不壓縮計畫，並在同事協助下僅完成原本構想的一半就倉促推出。也因為如此，今日的玩家普遍認為《異形戰士》是擁有巨大潛力但被平台限制的「未完成傑作」。

儘管如此，當年上市後依然受到硬派玩家盛讚，其極具張力的戰鬥節奏、視覺效果、誇張的Boss設計與寶石級音樂皆被視為Mega Drive時代的巔峰表現。部分批評則集中在極端的高難度、缺乏教學的操作機制與不尋常的遊戲方式，但多年來它逐漸被奉為16位元時代最代表「Treasure 風格」的作品之一。

隨著時間推移，《異形戰士》被收錄於日本PS2《SEGA AGES 2500》系列、全球版Wii Virtual Console與Steam平台，使更多新世代玩家能重新體驗這款曾經難以觸及的經典。對熟悉《Gunstar Heroes》或熱愛高速射擊遊戲的玩家而言，本作始終是極具象徵性的存在——是對技巧、反應與極限精神的純粹挑戰，也是90年代Treasure最具野心的代表作之一。

Alien Soldier, developed by Treasure and published by Sega for the Mega Drive in 1995, stands as one of the most ambitious and intense run-and-gun games of its era. Although it received physical cartridge releases only in Japan and PAL regions—and was accessible in North America solely through the Sega Channel—its limited distribution eventually helped it achieve cult status among dedicated action-game fans. Over the years, it has come to be regarded not only as a showcase of Treasure’s signature style, but also as one of the most extreme, skill-driven titles in the 16-bit generation.

The game takes place on the planet Sierra, where a powerful terrorist organization known as Scarlet rises under the violent leadership of a creature called Xi-Tiger. Scarlet aims to exterminate humanity, and at the center of this conflict is the protagonist Epsilon-Eagle, the group’s former leader who was nearly killed during an internal assassination attempt. Flinged into a breach in the space-time continuum and presumed gone forever, his consciousness later resurfaces inside a young boy with special abilities. When Xi-Tiger attacks a research facility and threatens a captured girl, the boy releases his rage and transforms back into Epsilon-Eagle’s full form. From that moment, the revived Epsilon—now split into distinct good and evil halves due to his time-space exile—begins a relentless pursuit of Xi-Tiger, driven by vengeance and the desire to end Scarlet’s rule once and for all.

Although the game opens with a long and somewhat confusing text crawl explaining this background, it never references the plot again once the gameplay begins. Treasure clearly understood that players weren’t coming to Alien Soldier for its lore—they were coming for its fierce, nonstop combat and brutally demanding mechanics.

Alien Soldier’s identity is defined by its “boss-rush” nature. The game features 25 stages and more than 26 bosses, but its traditional levels are extremely short, acting only as brief breathers between fierce battles. The true focus lies in the boss encounters, which combine lightning-fast movement patterns, dense projectile attacks, and aggressive pacing. Treasure’s combat philosophy, established in Gunstar Heroes, reaches a distilled and extreme form here, resulting in a game where every encounter tests precision, reflexes, and mastery of complex mechanics.

Epsilon-Eagle’s movement capabilities are unusually advanced for a 16-bit action game. He can run, dash, double jump, hover, cling to ceilings, and unleash a devastating high-speed attack called Phoenix Force, activated when the player uses a dash at full health. This move tears through enemies with explosive power and has become a signature highlight in expert gameplay videos.

Players can equip four weapons out of six available types, each featuring its own ammunition bar. Choosing the right combination at the beginning of the game greatly affects one’s strategy across the many boss battles. A unique counter mechanic allows players to convert enemy bullets into health—or in some cases reflect them back—if timed with perfect precision. Meanwhile, the ability to switch between “fixed fire” (stationary aiming) and “free fire” (moving while shooting) adds an additional layer of complexity, making Alien Soldier feel closer to a high-speed technical fighting game than a traditional shooter.

The game’s development story is as intense as its gameplay. After finishing Gunstar Heroes, Treasure began work on four different projects simultaneously, and Alien Soldier was placed under the direction of Hideyuki Suganami, who initially intended to create nearly the entire game by himself. His plans were incredibly ambitious, envisioning a massive, mechanically rich title overflowing with new ideas. However, the Mega Drive market was rapidly shrinking as the industry shifted toward 32-bit hardware. Faced with time pressure and platform decline, Suganami received help from other Treasure staff, and the project was released with only about half of his original vision realized. Even so, the finished game was crafted explicitly for “hardcore Mega Drive players,” a philosophy reflected in its unforgiving difficulty, scoring system, and precision-focused combat.

When Alien Soldier launched near the end of the Mega Drive’s life cycle in February 1995, critics noted its high difficulty and unique gameplay structure. Over time, retrospective reviewers have praised the game’s intense battles, striking visual presentation, energetic soundtrack, and uncompromising design. While some criticism remains directed at its steep learning curve, unconventional mechanics, and brutal difficulty, the game is widely recommended to fans of Treasure’s earlier titles and the run-and-gun genre as a whole.

Alien Soldier has been re-released several times, including on the Japanese PS2 Sega Ages 2500 series—which added regional variations and new features—as well as globally on the Wii Virtual Console and Steam. These re-releases have allowed modern players to experience a title that once felt inaccessible, giving new generations the opportunity to appreciate its unique blend of technical precision, action intensity, and pure 90s game design spirit.

Today, Alien Soldier is celebrated as one of Treasure’s boldest works: a game that pushes mechanical skill to its limits, delivering a relentless, exhilarating experience that remains unmatched in its genre. It stands as a testament to what the Mega Drive hardware could achieve in the hands of a studio obsessed with action, creativity, and pushing boundaries.