Let's say I have {avacyn} out making {gisela} indestructible, and my opponent plays a {gisela} of there own, what happens? Does the legendary rule apply taking them both out, or does my avacyn protect mine?
Legend rule takes it out by sending it to gy, not destroying, both giselas die
Indestructable does not bypass the Legend rule.
When a legendary creature of the enters the battlefield, the game state checks for another creature of the same name, if atleast two or more legendary creature are of the same name then all of them are put into the graveyard immediately.. Indestructability prevents destroy / damage effects from affecting creatures, but not the legend rule.
Outside of standard, {Mirror Gallery} will prevent the legend rule.
edit: Sacrifice would not be the appropriate term, but its similar, they just can't be responded to.
Confirmed. 🔨