I have a question about ruthless deathfang, but really it is just a question about triggered abilities period. Ok let's say my opponent has only deathfang on the board, and I force him to sacrifice a creature, since he only has the one deathfang on his battlefield, he's must sacrifice it, does he then get to make opponent sacrifice as well? I said he doesn't get to because the triggered ability only works if the creature is still on the battlefield when one of his creature is sacrificed. Since he has to sacrifice the deathfang, then there is no triggered ability because the deathfang is dead?
Ruthless Deathfang has an ability that triggers whenever its controller sacrifices a creature, even if that creature is Ruthless Deathfang itself (C.R. 109.5). This is because that ability is a leaves-the-battlefield ability, which triggers based on the state of the game immediately before the event of leaving the battlefield (C.R. 603.6c, 603.10, 603.10a, 701.16a). Moreover, a triggered ability, "[o]nce ... triggered", "exists ... independently of its source", so will get to resolve no matter what happens to that source (C.R. 112.7a).
Note that the rules provide no window for players to sacrifice permanents except as an incidental part of another action or event (such as to pay a cost or as part of an effect).
Note that the rules provide no window for players to sacrifice permanents except as an incidental part of another action or event (such as to pay a cost or as part of an effect).
EDIT (Sep. 29, 2018): Correct quotation.