Can the stack be set up in a way to get life from soul warden when the zombies and Scion etb?
If this is the case and I do receive 4 life.. can someone explain to me how this happens? I would think that soul warden hitting the graveyard would end her effect before the creatures etb but I've heard otherwise.
Assuming you control Dark Supplicant, Soul Warden and Rotlung Reanimator:
No. As part of activating Dark Supplicant's ability, you first put it on the stack before paying its cost (C.R. 602.2a; for paying costs, see C.R. 602.2b, 601.2h). If you sacrifice Soul Warden as you pay this cost, Soul Warden will leave the battlefield before the Dark Supplicant ability resolves, so that it won't be there by the time Scion of Darkness or the Zombie tokens, if any, enter the battlefield.
In general, if an ability (such as Rotlung Reanimator's ability) triggers as you pay the cost of a spell or ability (such as Dark Supplicant's ability), the triggered ability will go on the stack above that spell or ability (C.R. 405.2) and get to resolve before it (C.R. 116.4). In this case, if you sacrifice Rotlung Reanimator and two other Clerics to pay for Dark Supplicant's ability, Rotlung Reanimator's ability will trigger three times, once for Rotlung Reanimator and once each for the other Clerics, and this is the case even though the three creatures die at the same time (C.R. 700.4, 603.6c, 603.10, 603.10a).
The rule allowing you to order abilities (C.R. 603.3b) applies only if multiple triggered abilities you control are involved, but not if one activated ability and one triggered ability are involved.
The scenario is:
Dark Supplicant is activated and 3 clerics including Soul Warden are sacrificed.
Rotlung Reanimator is on the field.
Can the stack be set up in a way to get life from soul warden when the zombies and Scion etb?
If this is the case and I do receive 4 life.. can someone explain to me how this happens? I would think that soul warden hitting the graveyard would end her effect before the creatures etb but I've heard otherwise.
No. As part of activating Dark Supplicant's ability, you first put it on the stack before paying its cost (C.R. 602.2a; for paying costs, see C.R. 602.2b, 601.2h). If you sacrifice Soul Warden as you pay this cost, Soul Warden will leave the battlefield before the Dark Supplicant ability resolves, so that it won't be there by the time Scion of Darkness or the Zombie tokens, if any, enter the battlefield.
In general, if an ability (such as Rotlung Reanimator's ability) triggers as you pay the cost of a spell or ability (such as Dark Supplicant's ability), the triggered ability will go on the stack above that spell or ability (C.R. 405.2) and get to resolve before it (C.R. 116.4). In this case, if you sacrifice Rotlung Reanimator and two other Clerics to pay for Dark Supplicant's ability, Rotlung Reanimator's ability will trigger three times, once for Rotlung Reanimator and once each for the other Clerics, and this is the case even though the three creatures die at the same time (C.R. 700.4, 603.6c, 603.10, 603.10a).
The rule allowing you to order abilities (C.R. 603.3b) applies only if multiple triggered abilities you control are involved, but not if one activated ability and one triggered ability are involved.
See also this thread.