So I heard that killing multiple herald in a single game would have some benefits? Is that true? And if it's correct, what happens if I get disconnected from a game and come back. I do I keep the kill count?

heralds go up in tiers as you get them, visible by name
afaik the game will keep the count if you come back to the same one

T4 and T5 have a special TC class for Latent Sunders.
T1 T2 T3 can drop em too but they do it thru upgrading a Magic GC to Unique, then choosing Sunder over Gheeds. Hard path.

They can also drop all sorts of things and have a Unique factor thats better than bosses, which gives em better odds at uniques than bosses themselves.

i dropped from my game and the next herald was again terror,so i am guessing somehow it keeps the count.