1. The damage of Eclipse is dependent on the level of Lucent beam
2. If you have Eclipse yet did not put one single level on Lucent beam, the Eclipse will not produce any damage at all, just a pretty lightshow.
3. Many enemies would mean the beams will be distributed almost evenly.
4. If a single Hero is in your Eclipse’s AOE range he will not receive all of the beams only the maximum, depending on the level of eclipse (Ex. level 3 has a maximum of 4 eclipse)
5. Aghanims will increase the number of Lucent beams and the maximum beams that would hit the target. (Ex. level 3 would have 12 beams and a maximum of 5 beams hitting a single target.)
If you have a level 2 Eclipse it is better to cast it against 2 heroes. Cast it on 3 heroes when you reach level 3 Eclipse so as not to waste the extra beams.