Bolg of the North

If you don't control an Army, the Goblin Army token you create enters the battlefield as a 0/0 creature before receiving counters. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.

You can't sacrifice multiple creatures to deal damage multiple times.

Some cards refer to the "amassed Army." That means the Army creature you chose to receive counters, even if no counters were placed on it for some reason.

Amass Orcs works the same way, except you create a 0/0 black Orc Army creature token if you don't control an Army. And if the Army creature you chose isn't already an Orc, it becomes an Orc in addition to its other types. By combining cards with amass Orcs and amass Goblins, you can end up with a Goblin Orc Army.

The sacrificed creature's last known existence on the battlefield is checked to determine its power.

Bolg of the North's damage-dealing ability triggers only when you sacrifice a creature as a result of the instruction of its triggered ability. It won't trigger if you sacrifice a creature for any other reason.

Excess damage has been dealt to a creature if the damage dealt to it is greater than lethal damage. Usually, this means damage greater than its toughness, although damage already marked on the creature is taken into account.

In the rare case that you control multiple Army creatures (perhaps because you control a creature with changeling) while you amass Goblins, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't a Goblin, it becomes a Goblin in addition to its other types.

To amass Goblins N, if you don't control an Army creature, create a 0/0 black Goblin Army creature token. Then you choose an Army creature you control and put N +1/+1 counters on it. If that Army isn't already a Goblin, it becomes a Goblin in addition to its other types.

Bolg of the North has a reflexive triggered ability. When it enters the battlefield, its triggered ability goes on the stack without a target. While that ability is resolving, you may sacrifice another creature. If you do, a second ability triggers and you pick a target that will be dealt damage. This is different from other abilities that say "If you do . . ." in that players may cast spells and activate abilities before a creature is sacrificed and then again after the creature is sacrificed but before damage is dealt.

Some spells and abilities that amass Goblins may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't amass Goblins.