Big brothers of flamme sentinels and twin of rime sentinels. They have the ability to cast rapid short fire beam in series with a small aoe, they have fast energy shield, good resistances, and high dps.
A good choice for long term Raise Spectre, they can be enhanced via skills gems like spell echo, fast casting, controlled destruction...
Can be found in and act 3 or 8, in the Solaris temple.