D-Demon and attack phase

[CommanderUnderSorin]

At the beginning of combat, my opponent sacs a creature to  {Desecration Demon},
At the beginning of declaring attackers can  {Triton Tactics}, untap Demon so I can swing anyway?

[Giggle the Draco Genius]

Since Desecration Demon does not say that it can not attack if a creature is sacrificed, yes you can as long as you do it still in the beggining of combat phase and not after the declare attackers.

[Pleeb]

At the beginning of combat, my opponent sacs a creature to  {Desecration Demon},
At the beginning of declaring attackers can  {Triton Tactics}, untap Demon so I can swing anyway?

When combat starts, desecration demons ability goes on the stack. When it resolves, each opponent in turn order has an opportunity to tap the demon. Once it finishes and the demon is tapped, you will receive priority and can then untap the demon. All of this occurres during the beginning of attack phase. Once all players pass priority in a row, the game shifts to declare attackers phase, when it becomes too late to untap the demon.

[sithantic]

In this case, once he untaps it, could another creature be sac'd, since it's still in the beginning of combat phase?

[FustyDavorite]

In this case, once he untaps it, could another creature be sac'd, since it's still in the beginning of combat phase?

no. His ability only triggers once beginning of combat step begins. It will not be triggered once he's been untapped