(1) Although the gaming world sees you as a clan or guild, Shadowclan (or at least one of its members) must manifest itself in a traditional legal form in order to register and enforce trademark rights. (It's a little bit like the problem Zorro or Superman might have enforcing their trademarks.) In other words, Shadowclan must register with the USPTO as an individual (sole proprietor), partnership, LLC, or some for-profit or nonprofit variation. If the guild is a partnership (one way to handle it) then you're in a situation similar to a band of musicians who must decide who controls the rights to their name, and plan for what happens if original members leave and other members join.
(2) The Dear Rich staff has determined that there are no federal registrations for the term Shadowclan. However, that may soon change with the release of a new video game. In addition, it appears as someone is using the name on merchandise. Again, registration is the best step to take if you want to enforce rights. You must determine what type of services or goods are associated with your group and register accordingly. (This Nolo book may help.) You can only enforce rights against those who offer similar or related goods and services.
(3) A reader (thanks Kurt) has informed us that a guild, The Syndicate, has acquired a federal registration (No. 3027852) for "entertainment services, namely, an online gaming guild providing in game opportunities for proliferating game expertise and camaraderie among gamers supported by a web site featuring multimedia materials." That's good news as you have a precedent for registration.
(4) By the way, although a service mark is a solid choice, another consideration is to pursue a "collective mark." A common use of collective marks is to show membership in a union, association, or other organization. Collective marks are entitled to registration and the same federal protection as other types of marks. With a collective mark, you can prevent someone from identifying themselves as a member of your organization (if they're not.)