How would baron resolve being played by rift herald's desthknell effect?

Baron Nashor enters the Baron Pit, not your base — his "As you play me" replacement effect overrides the Deathknell's "to your base" instruction.

Yes — you can play Baron off Rift Herald's Deathknell. The Deathknell puts the play action on the Chain; Baron is still "played" from your hand, so his replacement effect (Rules 369–370) applies during that play. It replaces the event of him entering at your base with: add the Baron Pit token, then he enters there. This is the same interaction as with any play-location modifier — the Deathknell's destination instruction is what gets replaced (see Rule 369.3 example for Baron).

Two things to remember:

• Cost: Deathknell only ignores his Energy cost — you still pay Baron's Power cost (3 [C]). Rule 356.1.b.2; FAQ #10760.
• Scoring: Baron entering the empty Baron Pit counts as a Conquer, subject to the once-per-turn scoring limit (CR 449) and the Final Point requirement (CR 450.1.b).