hi, the party were fighting living ice crystals in last nights game and taking persistent cold damage each round. just checking I did this right.

the half orc went down on her turn to 0 hp,  then party killed an enemy next to the half orc, when the crystals die they do a death burst of 5' radius so as it was within 5' of a pc that was incapacitated the orc took 2 death save failures as any hit counts as a crit if within 5' ?  she then took cold damage which was a single failure as it was persistent damage or is that a crit too? I know the last one is irrelevant but would be nice to know. 

                                                                                                                     thanks Adam

The death burst of the Living Ice Crystals can only be a critical hit, if it uses an attack roll.

So, it probably would have been 1 failed death save for the death burst + 1 failed death save for the ongoing cold damage effect.

If the death burst is an attack with a d20 to hit roll, then it would have been a critical hit and 2 failed saves.

thank you for that, i will have a happy player now.

As Voras said the critical hit is only possible if an attack roll is made and not for automatic damaging effect or one occuring on a failed saving throw like a Magmin's Death Burst has.

But one way or another, if the damage equals or exceeds your hit point maximum, you suffer instant death though.