If a planesawalker, let's say Nissa, who Shakes the World gains indestructible from heroic intervention, then gets attacked that turn for sufficiently lethal combat damage which isn't being prevented in any way, is she "dying", or does she simply go to the grave as a state based action?
What if she's also a creature somehow (say via liquimetal coating + march of the machines), will going to the grave as a result of losing loyalty counters still create "when dies" triggers for something like grave pact? Can she no longer "die" as a creature with indestructible, or will that only prevent destroy effects?
Lastly, (and I'm sorry if this is a silly question) if attacked while also a creature, will Nissa's power (in this case 5) deal damage to the attacker(s) or she still unable to reciprocate?
If Nissa is a planeswalker, Nissa will go to the graveyard ("die") as a state-based action once she's dealt enough combat damage to reduce Nissa's loyalty to 0 (C.R. 704.5i, 700.4). This is true even if Nissa is also a creature or has indestructible. This is notably because that state-based action doesn't destroy that permanent.
More generally, a planeswalker that runs out of loyalty counters, whether by damage or otherwise, will go to the graveyard as a state-based action (C.R. 704.5i, 306.5c), even if that planeswalker is also a creature or has indestructible.
In general, if Nissa is a planeswalker creature, damage dealt to Nissa will remove that many loyalty counters from Nissa and place that much marked damage on her (C.R. 120.3c, 120.3e). If Nissa has indestructible and the amount of damage marked on her meets or exceeds Nissa's toughness, but Nissa also has no loyalty counters on her, one state-based action won't destroy Nissa (even though she has lethal damage) (C.R. 704.5g, 702.12b), but another will still send Nissa to the graveyard for having a loyalty of 0 (C.R. 704.5i, 306.5c). Because Nissa goes to the graveyard from the battlefield, she will "die" for the purposes of triggered abilities such as Grave Pact's (C.R. 700.4), which will trigger in this case if Nissa is a "creature you control".
Finally, if Nissa is being attacked while she's both a planeswalker and a creature, she won't assign combat damage unless she also blocks (only attacking and blocking creatures can assign combat damage [C.R. 510.1a], and a planeswalker that's being attacked is not necessarily either).
If a planesawalker, let's say Nissa, who Shakes the World gains indestructible from heroic intervention, then gets attacked that turn for sufficiently lethal combat damage which isn't being prevented in any way, is she "dying", or does she simply go to the grave as a state based action?
Both, Nissa "dies", which means that Nissa goes to the graveyard from the battlefield (C.R. 700.4), for the reasons given above.
What if she's also a creature somehow (say via liquimetal coating + march of the machines), will going to the grave as a result of losing loyalty counters still create "when dies" triggers for something like grave pact? Can she no longer "die" as a creature with indestructible, or will that only prevent destroy effects?
Nissa still "dies", which means that Nissa goes to the graveyard from the battlefield (C.R. 700.4), even if Nissa is also a creature, for the reasons given above. Thus, in general, abilities that trigger "whenever a creature [meeting certain criteria] dies" (such as found in Grave Pact) will trigger if Nissa is a creature and otherwise meets those criteria.
Lastly, (and I'm sorry if this is a silly question) if attacked while also a creature, will Nissa's power (in this case 5) deal damage to the attacker(s) or she still unable to reciprocate?
Nissa will assign combat damage only if she's an attacking or blocking creature (C.R. 510.1a).
As long as Nissa goes to the grave from the field, she "dies".
Cause of death is irrelevant: stuff will trigger.
From the Glossary:
A creature or planeswalker “dies” if it is put into a graveyard from the battlefield. See rule 700.4.
700.4. The term dies means “is put into a graveyard from the battlefield.”
As for being attacked while she's a creature, here's the ruling under Gideon Jura:
Gideon Jura may block (since he’s a creature). He may block any eligible attacking creature, including one that’s attacking him! During combat, he behaves as an attacked planeswalker and/or a blocking creature, as appropriate. For example, he deals combat damage to any creatures he’s blocking, but he doesn’t deal combat damage to any unblocked creatures that are attacking him.
What if she's also a creature somehow (say via liquimetal coating + march of the machines), will going to the grave as a result of losing loyalty counters still create "when dies" triggers for something like grave pact? Can she no longer "die" as a creature with indestructible, or will that only prevent destroy effects?
Lastly, (and I'm sorry if this is a silly question) if attacked while also a creature, will Nissa's power (in this case 5) deal damage to the attacker(s) or she still unable to reciprocate?
I used to be a demigod, but now I'm an omnimage
More generally, a planeswalker that runs out of loyalty counters, whether by damage or otherwise, will go to the graveyard as a state-based action (C.R. 704.5i, 306.5c), even if that planeswalker is also a creature or has indestructible.
In general, if Nissa is a planeswalker creature, damage dealt to Nissa will remove that many loyalty counters from Nissa and place that much marked damage on her (C.R. 120.3c, 120.3e). If Nissa has indestructible and the amount of damage marked on her meets or exceeds Nissa's toughness, but Nissa also has no loyalty counters on her, one state-based action won't destroy Nissa (even though she has lethal damage) (C.R. 704.5g, 702.12b), but another will still send Nissa to the graveyard for having a loyalty of 0 (C.R. 704.5i, 306.5c). Because Nissa goes to the graveyard from the battlefield, she will "die" for the purposes of triggered abilities such as Grave Pact's (C.R. 700.4), which will trigger in this case if Nissa is a "creature you control".
Finally, if Nissa is being attacked while she's both a planeswalker and a creature, she won't assign combat damage unless she also blocks (only attacking and blocking creatures can assign combat damage [C.R. 510.1a], and a planeswalker that's being attacked is not necessarily either).
See also this thread.
Therefore:
Both, Nissa "dies", which means that Nissa goes to the graveyard from the battlefield (C.R. 700.4), for the reasons given above.
Nissa still "dies", which means that Nissa goes to the graveyard from the battlefield (C.R. 700.4), even if Nissa is also a creature, for the reasons given above. Thus, in general, abilities that trigger "whenever a creature [meeting certain criteria] dies" (such as found in Grave Pact) will trigger if Nissa is a creature and otherwise meets those criteria.
Nissa will assign combat damage only if she's an attacking or blocking creature (C.R. 510.1a).
EDIT (Dec. 12): Correctness edit.
Cause of death is irrelevant: stuff will trigger.
A creature or planeswalker “dies” if it is put into a graveyard from the battlefield. See rule 700.4.
700.4. The term dies means “is put into a graveyard from the battlefield.”
As for being attacked while she's a creature, here's the ruling under Gideon Jura:
Gideon Jura may block (since he’s a creature). He may block any eligible attacking creature, including one that’s attacking him! During combat, he behaves as an attacked planeswalker and/or a blocking creature, as appropriate. For example, he deals combat damage to any creatures he’s blocking, but he doesn’t deal combat damage to any unblocked creatures that are attacking him.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules