Crystal Maiden

Rylai the Crystal Maiden is a ranged intelligence Hero used as a hard support and disabler. She is perhaps best known for her powerful global mana regeneration aura. This aura allows mana dependent allies to excel during the early to mid game and the laning phase, and allows her to constantly activate her own spells without needing to worry much about the mana costs. She also has a high amount of early game presence with just a few points in her Crystal Nova and Frostbite abilities. Crystal Nova is a powerful area-of-effect nuke that slows both attack and movement speeds of enemies within an area for several seconds, while Frostbite encases an enemy in a block of ice for several seconds, immobilizing it and doing moderate damage per second. Combining these two abilities together along with a laning partner's own spells often results in the quick death of an enemy hero. Although her strong early game presence is quickly lost due to her extreme frailty and poor mobility, she can still inflict a heavy amount of damage in teamfights later on if she is able to channel her deadly ultimate, Freezing Field. Her Arcane Aura, strong disabling and nuking prowess, relative ease of usage, and almost complete lack of item dependence makes her a reliable support caster that can be useful in any team.

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.
 * Generally taking a single point in Arcane Aura early is sufficient enough for the whole team, as it provides nearly twice the mana regeneration as a Ring of Basilius does, on global range. Other skill points should be invested to max Crystal Nova and Frostbite asap.
 * Since 6.76 this bonus is doubled on Rylai herself, granting a whopping +5 bonus mana/s at max level.
 * 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, to do so is to gild the lily; Freezing Field at level 3 is staggeringly powerful even without a scepter and it's wiser to build items that improve her chances of channeling it.

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.