Crystal Maiden

Rylai the Crystal Maiden is a ranged intelligence Hero mostly used as a support and disabler known best for her powerful global mana regeneration aura. This aura allows mana dependent allies to excel during the early or laning phase. She can also go out on an offensive, investing on Crystal Nova and Frostbite abilities earlier in the game to create a more devastating presence. Her powerful area nuke and slow coupled with her moderately long disable even at its first level makes her a tremendous roamer in the early game while still having a dominant power, although of less quality in the later stages. The Arcane Aura combined with her slow and disable abilities make her one of the more common support Heroes.

Tips

 * Crystal Nova is a large area of effect nuke that also slows your enemies. You can hit enemies from 1100 units away with it (700 casting range + 400 radius).
 * Frostbite is a great disable which prevents the target from moving or attacking. It also interrupts channelling spells and teleports. Unfortunately, it does not prevent enemy heroes from casting spells.
 * Arcane Aura used to be prioritized as it benefits the whole team greatly, especially one with a strong caster/ganker line-up. But the introduction of Arcane Boots has made this skill somewhat less useful.
 * Since 6.76 this bonus is doubled on Rylai herself, granting a whopping +4 bonus mana/s.
 * Freezing Field is an incredibly strong spell. However, it can be unpredictable and usually gets interrupted by your enemies very quickly. Freezing Field consumes a lot of mana, so use it sparingly.
 * Many players choose not to skill Freezing Field until quite late in the game, as it is a situational spell that is most effective in large clashes; and its use consumes a huge amount of mana. However, its slow and gigantic (if unpredictable) damage can be very useful in early game, especially against enemies that lack the means to interrupt it. Let your decision be informed by enemy lineup as well as laning composition.
 * Items like Black King Bar and Shadow Blade are viable options to prevent your opponents from cancelling your ultimate, but Crystal Maiden usually won't have that much gold to spend.
 * Crystal Maiden can either babysit her team's carry on dual/triple lanes, or roam between the lanes to gank enemies.
 * Always ward the woods. Wards should be bought over and over again to provide map control through the whole game.
 * There is a temptation to use Crystal Nova to push lanes, but owing to its mana cost and long cooldown this isn't a good idea.
 * If you have a few coins to spare - always buy items that boost your survivability, or get yourself an escape/positioning mechanism like Blink Dagger or Force Staff.
 * Freezing Field must be used at the right time to avoid interruption and being focused. Generally speaking, in an encounter it is best used after the initial nuking phase; when the enemy's stuns are on cooldown (building a Black King Bar will provide immunity to stuns; but Crystal Maiden does not typically get much gold and this can be very difficult to build in a game where your team is not getting a lot of kills). If your team has an initiator with the capability to disable a group of enemies, such as Tidehunter or Enigma, Freezing Field should be used in synchronisation with their mass disable. Using Freezing Field very often results in Rylai's death, and she rarely is able to channel it for its full duration, but if she can maintain it even for a few seconds it will be useful for her team.
 * Although Freezing Field's damage can be upgraded with Aghanim's Scepter, its primary use is its slow rather than its damage and considering Rylai's low gold income, building Scepter is typically not a good idea; her ult would be more useful if instead she built Black King Bar and/or Blink Dagger and typically of a support hero she normally must choose only a couple of luxury items.

Trivia

 * Crystal Maiden's mana regen aura used to be called Brilliance Aura.
 * In early versions of DotA, Crystal Maiden and Lich used to share the same Frost Nova ability. However, in later updates, Crystal Maiden's Frost Nova was changed to Crystal Nova, turning it into a free-cast area-of-effect ability, while Lich kept the original effects.
 * Her Frostbite icon is similliar to Treant's ult, Overgrowth.